Hi Dan,
Which folder is missing? Is it a default mailbox folder (Inbox, Sent Items folder, etc.) or a custom folder?
Have you checked whether this folder was accidentally moved under another folder?
Can you still see the folder if we check it from OWA?
We may first try to start Outlook with /resetfolders switch to restore the default folders and see whether it helps. To do this, close Outlook, press Win key + R to open the Run command, type
outlook /resetfolders and press Enter.
Regards,
Steve Fan
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