OUTLOOK WON'T OPEN

A current Microsoft 365 service degradation may affect connections, but it does not specifically confirm that classic Outlook cannot open, so end any stuck Outlook process and test the application in Safe Mode while using Outlook on the web temporarily.^1^

1. Close a hidden Outlook process

1. Press Ctrl + Shift + Esc to open Task Manager.
2. Find Microsoft Outlook or Outlook.
3. Select it, then choose End task.
4. Close Task Manager and open Outlook again.

This addresses cases where Outlook remains open invisibly in the background.

2. Test classic Outlook in Safe Mode

Safe Mode starts Outlook without add-ins:

1. Press Windows key + R.
2. Type: outlook /safe
3. Select OK, accept the default profile, and enter the account password if requested.^2^

If Outlook opens in Safe Mode:

1. Select File → Options → Add-ins.
2. Beside Manage, select COM Add-ins → Go.
3. Record or photograph which add-ins are selected.
4. Clear their check boxes, then select OK. Disable them—do not use Remove.
5. Restart Outlook normally.
6. Re-enable the add-ins individually, restarting Outlook after each one, until the problematic add-in is identified.

3. If Safe Mode also fails

Run Microsoft’s automated startup check:

1. Open the Windows Get Help application on the same computer.
2. Search for classic Outlook startup troubleshooter.
3. Run the troubleshooter and apply its recommended correction.

It requires Windows 10 or later and applies to classic Outlook, not new Outlook.^3^

If the troubleshooter cannot repair it, create a separate Outlook profile:

1. Close Outlook.
2. Open Control Panel → Mail → Show Profiles → Add.
3. Enter a new profile name and add the email account.
4. Start Outlook and select the new profile.

If that profile opens, the original profile is damaged. Do not remove the old profile until any locally stored data has been backed up.^2^

Temporary access

While testing the Windows application, open the mailbox through Outlook on the web in a browser. Refreshing the desktop application again is unnecessary because it has already made no difference.

References

1. Service Health Status - Microsoft
2. I can’t start Microsoft Outlook or receive the error “Cannot start Microsoft Outlook. Cannot open the Outlook Window” | Microsoft Support
3. Classic Outlook troubleshooters | Microsoft Support