You were invited to join another user’s Plex Home.
There is a warning dialog, which tells you explicitly that joining another user’s Home will result in the loss of your own Plex Home and all managed users within it.
And this is unfortunately exactly, what ensued when you accepted the invitation.
You need to leave this Plex Home. (Settings - Family & Friends - Home Users)
There is (almost) no way to restore your previous managed users.
If you are very computer savvy and know what a SQL query is, you can try this user’s suggestions: Added an existing user and all my managed users disappeared!?!?!?!?!
to re-assign your and your managed user’s ‘played’ history after you re-created your managed users.
