Outlook won’t open: how to fix it in Windows 11

Applies to: classic Outlook for Microsoft 365, Outlook 2024 and Outlook 2021, plus the new Outlook for Windows, on Windows 11 (most steps also work on Windows 10). Use it when Outlook doesn’t start at all, shows Cannot start Microsoft Outlook. Cannot open the Outlook window, hangs on the loading screen or closes straight after it opens.

The order below follows Microsoft’s support article for this error, from the least disruptive step to the most. If Outlook does open but keeps asking you to sign in, our password prompt guide is the better fit. If you want to put Outlook’s settings back to defaults on purpose, see how to reset Outlook to default settings. Where a step here is our suggestion rather than Microsoft guidance, we say so.

Match your symptom

What you see	Likely cause	Start with
Nothing happens when you click the icon, or Outlook flashes and disappears	A stuck Outlook process, an add-in or a damaged profile	Before you start, then Fix 2
“Cannot start Microsoft Outlook. Cannot open the Outlook window”	Add-in, navigation pane settings or the profile	Fix 2, Fix 3, Fix 4
Outlook only opens in safe mode	A faulty or outdated add-in	Fix 2
Outlook hangs on the loading screen or shows Not Responding at start	Damaged data file, a POP or .pst profile issue, or security software	Fix 5, Fix 7, Known issues
Error about a data file or a .pst or .ost file	Damaged Outlook data file	Fix 5
Outlook crashes at start after an update	Damaged program files or a known issue	Fix 6, Known issues
The new Outlook window stays blank or won’t launch	App data problem in the new Outlook	Fix 8

Before you start: close stuck copies and check which Outlook you have

A copy of Outlook that didn’t close properly can stop a new one from starting, so clear that first.

• End leftover Outlook processes. Press Ctrl + Shift + Esc to open Task Manager. On the Processes tab, look for Microsoft Outlook or OUTLOOK.EXE, select it and choose End task. Only end Outlook, not other entries you don’t recognise. Then start Outlook again.
• Restart the PC. A restart clears anything still holding Outlook’s files, such as a sync or backup tool. This is our suggestion, but it’s quick and often enough.
• Check which Outlook you’re opening. Windows 11 can have both the classic Outlook and the new Outlook installed. The new Outlook has a New Outlook toggle in the top corner and is listed as Outlook (new) in your apps. Fixes 1 to 7 are for classic Outlook; Fix 8 is for the new one. If you were moved to the new Outlook and want the old one back, see our classic Outlook download guide.
• Older Office versions. Microsoft notes that Office 2016 and Office 2019 are no longer supported for connecting to Microsoft 365 services. If you use one of those with a Microsoft 365 mailbox, startup and connection problems may not get fixes.

Fix 1: Run the classic Outlook startup troubleshooter

Microsoft offers an automated troubleshooter for classic Outlook that won’t start. According to Microsoft’s article, it’s meant for Microsoft 365 business customers, needs Windows 10 or later, and must run on the same PC where Outlook is installed.

1. Open Microsoft’s support article I can’t start Microsoft Outlook on the affected PC and select the Start button for the troubleshooter.
2. If your browser asks whether to open Get Help, select Open.
3. Follow the prompts in the Get Help window. It checks common startup causes and can apply fixes for you.

If you use Outlook with a personal Microsoft account, or the troubleshooter finds nothing, carry on with the manual steps. They cover the same ground.

Fix 2: Start Outlook in safe mode and switch off add-ins

Safe mode starts Outlook without add-ins and some customisations. It’s the fastest way to tell whether an add-in is to blame. Microsoft’s article mentions an outdated ABBYY FineReader add-in as one known cause of the startup error.

1. Press Windows key + R to open Run.
2. Type outlook /safe and select OK.
3. In Choose Profiles, keep the default profile and select OK. Enter your password if asked.

In safe mode, the top of the window shows a safe mode label, and Microsoft notes that the Outlook taskbar icon carries a small caution symbol. If Outlook opens this way, an add-in is the likely cause:

1. Go to File > Options > Add-ins.
2. At the bottom, make sure Manage shows COM Add-ins and select Go.
3. Write down or screenshot which add-ins are ticked, so you can put them back.
4. Clear all the ticks and select OK. Don’t use Remove: that deletes the add-in instead of just turning it off.
5. Choose File > Exit, then start Outlook normally.
6. If it opens, turn the add-ins back on one at a time through the same dialog, restarting Outlook after each. When Outlook stops opening normally, the last add-in you enabled is the culprit. Microsoft points out that more than one add-in can be involved, so keep checking the rest.

Once you know the add-in, look for an update from its maker or leave it off. If Outlook won’t start even in safe mode, add-ins aren’t the main problem; move on to Fix 3.

Fix 3: Reset the navigation pane

The navigation pane is the left-hand column with your folders and the Mail, Calendar and People icons. Damaged settings for it can stop Outlook from opening its main window. Resetting it removes any customisations you made to that pane, but nothing else.

1. Close Outlook completely.
2. Press Windows key + R.
3. Type outlook.exe /resetnavpane (with the space) and select OK.

These settings are stored on each PC, so if the same problem appears on another computer, run the command there too.

Fix 4: Try a new Outlook profile

Your Outlook profile holds your account settings and the link to your data files. When it’s damaged, Outlook can fail to open no matter what else you try. Creating a second profile is a safe test because it doesn’t touch the old one.

1. Hold Shift while you start Outlook, or press Windows key + R and run outlook.exe /profiles. Both open the profile picker without needing Outlook to load.
2. Select Options. To choose a profile at every start, tick the option to prompt for a profile.
3. Select New, type a name for the profile and select OK.
4. Add your email account and finish the setup.
5. Start Outlook and pick the new profile from the list.

If Outlook opens with the new profile, the old one was damaged. For Microsoft 365, Exchange and Outlook.com accounts, the mail is on the server and downloads again. Before you remove the old profile, make sure you know where any .pst files from POP accounts or local archives are stored, because Microsoft warns that removing a profile deletes its offline cached content. You can add those .pst files to the new profile afterwards.

Fix 5: Repair the Outlook data files

Outlook keeps mail in data files: .ost files are an offline copy of a server mailbox, while .pst files hold POP mail and archives. A damaged file can make Outlook hang or refuse to start. Microsoft’s Inbox Repair tool, SCANPST.EXE, checks them.

Back up first. Copy the .pst file somewhere safe before you run the tool. Microsoft notes that some badly damaged items may not be recoverable, and the tool changes the file it repairs.

1. Exit Outlook.
2. Open SCANPST.EXE. For Microsoft 365 and recent versions it’s usually in C:\Program Files\Microsoft Office\root\Office16 or, for 32-bit Office, C:\Program Files (x86)\Microsoft Office\root\Office16. If it isn’t there, search the Microsoft Office folder for it.
3. Select Browse, pick your .pst file and select Start.
4. If errors are found, select Repair. You may need to run the tool more than once.
5. Start Outlook with the profile that uses the repaired file.

Repaired items can end up in a Lost and Found folder or a Recovered Personal Folders file; move what you need back into your normal folders. For an Exchange or Microsoft 365 mailbox, Microsoft says you can delete the .ost file instead and Outlook builds a new one from the server the next time it opens. If your mailbox is very large and Outlook later shows memory errors, our guide to Outlook out of memory or system resources covers that.

Fix 6: Update and repair Office

If none of the above helps, Outlook’s program files may be damaged, often after an interrupted update. Install pending updates first, then repair.

• Windows and Office updates. Open Settings > Windows Update and install what’s offered, then restart. Several 2026 startup problems were fixed through updates (see Known issues). If another Office app such as Word opens, you can also use File > Account > Update Options > Update Now there.
• Repair Office. Right-click Start and open Installed apps (called Apps and Features on older builds). Find your Microsoft 365 or Office entry, open the … menu, choose Modify and select Online Repair, then Repair.

Microsoft describes Quick Repair as faster but limited to finding and replacing corrupted files, and recommends Online Repair to make sure everything is fixed. Online Repair repairs the whole Office suite, needs an internet connection and may ask you to restart. Your mail and settings stay where they are.

Fix 7: Check antivirus and security add-ins

Security software that scans email often loads its own module into Outlook. If that module misbehaves, Outlook can crash or stop responding while it starts. Microsoft is currently investigating crashes linked to one antivirus product (see Known issues).

• Update your security software and restart.
• In safe mode (Fix 2), look for the security product’s add-in in the COM add-ins list and turn it off as a test.
• If Outlook opens normally without it, contact the security vendor for a fixed version rather than leaving protection switched off.

This section is our suggestion based on Microsoft’s known-issue notes; don’t uninstall or disable your antivirus permanently to make Outlook work.

Fix 8: The new Outlook won’t open

The new Outlook for Windows is a different app. It doesn’t use profiles, add-in dialogs or the startup switches above, so none of Fixes 2 to 5 apply. Microsoft’s fix for it is a repair or reset from Windows settings.

1. Right-click Start and select Installed apps.
2. Search for Outlook and find Outlook (new).
3. Open the … menu and choose Advanced options.
4. Select Repair and try opening Outlook again.
5. If it still won’t open, come back and select Reset.

Before you reset: Microsoft says Repair keeps the app’s data, but Reset deletes it. After a reset, you sign in again and the new Outlook downloads your mail from your accounts’ servers. A restart may be needed afterwards.

The new Outlook also needs to reach Microsoft’s servers to sign in. As a simple check of our own, see whether Outlook on the web opens in a browser with the same account. If the web version fails too, the problem is with the account or the service, not the app on your PC. And if you rely on features the new app doesn’t have, you can go back to the classic version with our classic Outlook guide.

Known issues in 2026

We checked Microsoft’s page Fixes or workarounds for recent issues in classic Outlook for Windows on November 8, 2026 (the page says it was last updated in November 2026). These 2026 entries relate to Outlook not starting or not responding:

• Classic Outlook crashes and opens in safe mode (from March 12, 2026). Marked Fixed. If you saw safe mode prompts around that time, update Office (Fix 6).
• POP and .pst profiles hang after the January 13, 2026 Windows update. Microsoft’s known issue page (updated February 3, 2026) says Outlook could show Not Responding and wouldn’t reopen until it was ended in Task Manager or the PC restarted, especially with .pst files stored in OneDrive. It’s marked fixed by Windows: install updates in Settings > Windows Update. If sync problems remain, Microsoft suggests repairing the profile and running SCANPST.EXE (Fixes 4 and 5).
• Outlook crashes with Kaspersky antivirus. Microsoft’s known issue page (dated June 26, 2026) is still marked Investigating. It links the crashes to the Kaspersky module mcou.dll and advises affected users to contact Kaspersky support. It doesn’t say whether the crash happens at startup.

On the matching page for the new Outlook for Windows, none of the 2026 entries describe the app failing to open, as of November 8, 2026.

Related guides

• How to reset Outlook to default settings (classic and new Outlook)
• Classic Outlook download: how to get it back on Windows 11 and 10
• Outlook keeps asking for password: how to stop it
• Outlook cannot display the folder, out of memory or system resources: how to fix it
• Outlook not receiving emails: how to fix it
• How to repair Microsoft Office (Quick Repair and Online Repair)

Official sources

• Microsoft: I can’t start Microsoft Outlook or receive the error Cannot start Microsoft Outlook
• Microsoft: Repair Outlook (new Outlook for Windows)
• Microsoft: Fixes or workarounds for recent issues in classic Outlook for Windows