https://forum.audacityteam.org/t/audacity-is-open-already-and-wont-open-solved/45617/1
Also, scan down on this post for a lock file.
https://askubuntu.com/questions/916841/invisible-audacity-session-running-wont-let-me-start-audacity
Your symptoms don’t exactly fit that, so it could be as simple as you ran out of space on your drive. As you edit, Audacity many times tries to save the whole show as a backup-UNDO. It can burn through drive space in a hurry, particularly if you’re trying to edit a multi-hour show.
Koz