You have most likely changed the email address or the authentication provider.
Log out of the Plex web client (top, right icon)
then log back in, using the same credentials and login method that you used to log in the server.
If you cannot remember which you used, stop and then restart Plex server. Wait 2 minutes. Then fetch the Plex Media Server.log manually https://support.plex.tv/articles/200250417-plex-media-server-log-files/
and search for the @ character in it. The first occurence should show the email address that is used for the server.
If that is unsuccessful, you’ll have to reclaim your server: You need to reclaim your server, but don't dare to touch the Windows Registry
