Same situation here, on multiple servers running Ubuntu and Plesk. Suddenly all database interaction fails. Restarting the server doesn't help. I had to reinstall everything and restore a backup.
This happened on multiple servers all in the same week and I see other users experiencing the same...
I had exactly the same issue as topic starter on Ubuntu.
Suddenly all database interaction fails and mariadb cannot be restarted. Happend on multiple vps'ses.
Plesks repair function didn't work either. I had to reinstall everything and restore a backup.