[Plagiarizing myself with a shameless copy/paste]
This could be caused by several things, but fairly high on the list is a corrupt database. You can pretty easily check this by forcing PMS to create a new one. Try renaming your Plex data folder (temporarily) so that a new one will be created. For example, rename:
/Users/{username}/Library/Application Support/Plex Media Server
to:
/Users/{username}/Library/Application Support/Plex Media Server.bak
Then try to start PMS. If it works, you can move your old directory back (you’ll have to delete the newly-created one, make sure to stop PMS first) and try to repair/restore you database. These two support articles cover that:
