office 365 wont open

If this is Outlook on Windows, restart the PC once and then use the matching new Outlook or classic Outlook steps below; if it is on a Mac, these Windows procedures do not apply.

New Outlook for Windows

If the application is labelled Outlook (new):

1. Press Ctrl + Shift + Esc to open Task Manager.
2. Select Microsoft Outlook, then select End task.
3. Close Task Manager and start Outlook again. This clears a background Outlook process that may be preventing another window from opening.^1^

If it still does nothing, reset its temporary data:

1. Press Windows + R.
2. Enter %localappdata%\Microsoft and press Enter.
3. Delete the Olk and OneAuth folders.
4. Start Outlook again. Outlook recreates these temporary folders; this does not delete mailbox email.^2^

If that fails, reinstall new Outlook:

1. Open Start, find new Outlook for Windows, expand its options, and select Uninstall.
2. Open Microsoft Store, search for new Outlook, and select Get.
3. Open it and add the email account if it is not restored automatically.^1^

Classic Outlook for Windows

Test whether an add-in is preventing startup:

1. Press Windows + R.
2. Enter Outlook /safe, then select OK.
3. Select the default profile if prompted.

If Outlook opens in Safe Mode:

1. Go to File → Options → Add-ins.
2. Beside Manage, select COM Add-ins → Go.
3. Record the enabled add-ins, then clear their checkboxes. Disable them; do not select Remove.
4. Restart Outlook normally.
5. Re-enable each add-in individually, restarting Outlook after each one, until the problem returns.^3^

If Safe Mode also fails, repair Microsoft 365:

1. Search for and open Control Panel.
2. Select Programs → Programs and Features.
3. Right-click Microsoft 365, then select Change → Online Repair.
4. Complete the repair and try Outlook again.^4^

If repair does not help, create a test profile without deleting the existing one:

1. Open Control Panel → User Accounts → Mail → Show Profiles.
2. Select Add, name the profile, and enter the email account details.
3. Hold Shift while starting Outlook.
4. Select the new profile and choose OK.

If Outlook opens with the new profile, the original profile is the problem. Do not remove the old profile until any locally stored data has been backed up; removing a profile also removes its associated local data files.^4^

While troubleshooting, the mailbox can be accessed through Outlook on the web at https://outlook.office.com.^5^

If these steps do not apply, provide the operating system (Windows or Mac), whether the app is new Outlook or classic Outlook, and whether Outlook appears in Task Manager after clicking its icon.

References

1. Outlook for Windows not responding, hangs, freezes, or stops working
2. Outlook (New) 365 suddenly will not open. - Microsoft Q&A
3. I can’t start Microsoft Outlook or receive the error “Cannot start Microsoft Outlook. Cannot open the Outlook Window” | Microsoft Support
4. Outlook crashes or stops responding when used with Office 365 | Microsoft Support
5. Microsoft 365 Enterprise Outlook won't launch - Microsoft Q&A