You could take a look into the Windows registry,
or in the server log files ~3 minutes after starting Plex server.
This should tell you which username/email address was used to claim your server.
Here is how to access the server’s log files manually: https://support.plex.tv/articles/200250417-plex-media-server-log-files#toc-1
Check your email inbox for mail from plex.tv. look in the Spam/irrelevant folders as well. It might be that someone got access to your user account and changed the associated email address. When that happens, plex is sending a warning per email with a link to revert this change.
Once you have found out the old email (given that the account was NOT capered), you can reset the plex account association of your existing server using this procedure: You need to reclaim your server, but don't dare to touch the Windows Registry
Don’t miss to read the linked procedure on how to now claim a server on Windows.
