Thank you for reaching Microsoft Community, David Stanley!
This is a common issue that many users experience, especially after increasing storage or switching to a new subscription plan.
Here sometimes to fixes it.
Check Folder Pane Settings. Sometimes the folder list is simply hidden.
Go to the View tab in Outlook.
Click Folder Pane and select Normal or Minimized to restore visibility.
Use Navigation Pane. If the Folder Pane is not the issue.
Go to View look for Navigation Pane and ensure Normal is selected.
Reset View Settings
Custom views may hide the folders.
Close Outlook.
Press Windows + R, then pop up box will showed up then type outlook.exe /cleanviews, and press Enter.
Or alternatively, in Outlook, go to View then Reset View.
Search for the Folder
You may have accidentally moved it.
Right click your mailbox name goto Data File Properties then Folder Size.
Look through the list to find the missing folder and its location.
Repair Outlook Installation
If none of the above is not working try this settings.
Go to Control Panel then Programs and look for Programs and Features.
Select Microsoft Office, Outlook, Repair
Tips
Use Ctrl + 6 to switch to folder view in Outlook desktop.
Check Inbox Rules Go to Settings goto Mail then Rules to ensure emails are not being auto moved or deleted.
Sync Issues If using IMAP, right click your inbox under IMAP folders look for Query then Subscribe to missing folders.
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Best Regards,
Lucky Charm | Microsoft Community