Microsoft Office 365 Outlook won't open unless I reboot computer

Try these steps in order for classic Outlook for Windows:

1. Start Outlook in safe mode.
   • Press Windows key + R.
   • Type Outlook /safe and select OK.
   • If Outlook opens, the issue is likely caused by an add-in.
2. Disable all COM add-ins.
   • In Outlook, go to File > Options > Add-ins.
   • At the bottom, confirm Manage shows COM Add-ins, then select Go.
   • Clear all selected check boxes, then select OK.
   • Restart Outlook normally.
   • If Outlook works, re-enable add-ins one at a time until the problem returns. That identifies the faulty add-in.
   • Do not use Remove during this test. If administrative rights are available, that can delete the add-in instead of just disabling it.
3. Reset the navigation pane.
   • Close Outlook.
   • Press Windows key + R.
   • Type Outlook.exe /resetnavpane and select OK.
   • This removes navigation pane customizations.
4. Create a new Outlook profile.
   • A damaged profile can prevent Outlook from opening correctly.
   • Create a new profile, then when Outlook starts, choose the new profile in the Choose Profile dialog.
   • If Outlook opens normally with the new profile, the original profile was corrupted.
   • If removing an old profile later, back up data files first. Removing a profile deletes offline cached content.
5. Repair Microsoft 365.
   • Open Control Panel.
   • Select Uninstall a program under Programs.
   • Right-click the Microsoft 365 installation and select Change > Online Repair.
6. If Outlook is hanging at loading profile or only works after a reboot, check for leftover Office processes.
   • Right-click the taskbar and open Task Manager.
   • Under Processes, locate Office processes.
   • Select each Office process and choose End task.
   • Then try Outlook again.
   • If this matches the known loading profile issue, restarting Outlook a couple of times can also help it pick up the service change.
7. If Outlook still does not open, run the classic Outlook startup troubleshooter or advanced diagnostic troubleshooter from Get Help on the same Windows PC.
   • These tools apply to classic Outlook and require Windows 10 or higher.

References:

• I can’t start Microsoft Outlook or receive the error “Cannot start Microsoft Outlook. Cannot open the Outlook Window” | Microsoft Support
• Outlook won’t open or is stuck at loading profile | Microsoft Support
• Outlook crashes or stops responding when used with Office 365 | Microsoft Support