If you or someone you know recently had an account compromised by replying to a phishing email, the scammers may have created a message rule that deletes the victim’s incoming email so that people cannot easily send them a message saying “Hey! There is something wrong with your email!”
To fix this:
1) Sign into the O365 Web Portal (not a phone, computer, or other device).
2) Click the Mail tile.
3) From the Gear menu in the upper right, click it, towards the bottom under “Your app settings” choose Mail.
4) Expand the triangle next to Mail on the left side (if it isn’t already), then choose “Automatic Processing”, and look at “Inbox and sweep rules”.
5) There will likely be a rule in there to delete/move messages which should be removed.
Deleted Messages can be found in the “Deleted Items” in the main mail window. If they are not visible there, click on the words “Recover deleted items” and they may be able to be recovered.