Stopped receiving emails in Gmail but can still send

Check whether an old phone or PC still has the account set up with POP. If some forgotten copy of Outlook is downloading by POP with the delete option on, it pulls the mail off the server before you ever see it. Sending would work fine, same as you describe.

Check your storage first. The 15 GB is shared with Drive and Photos, and when it fills up Gmail keeps letting you send but incoming mail bounces back to the sender. Phone backups eat it quietly. If storage is fine, look in Settings > Filters and Blocked Addresses and the Forwarding tab. A filter that skips the inbox or a forward set to delete the copy would look exactly like this. Search All Mail too, the messages may be arriving and just not landing in the inbox.