Start with the right checks.
First establish one example: sender, approximate date and subject. A concrete message is easier to trace than the impression that everything has vanished.
What to do next.
Confirm that you opened the correct account.
Search for the example message and check the current filter or focused inbox view.
Review expected folders, Junk, Archive and Deleted Items.
Look for an unexpected inbox rule or forwarding change, recording it before asking for help.
Check the result.
Confirm where the example message went and whether new messages behave normally. Keep the investigation narrow until the cause is understood.
Stop and get help if…
Rules changed unexpectedly, several important messages are absent or recovery is needed. Missing mail can have several causes; do not assume either compromise or accidental deletion without evidence.