I suffered this very same error last night after the MySQL server crashed. I was going to restore from a backup also but I thought I'd give the new forum's trouble-shooter a go first. I'm happy to say that it helped resolve the issue, as follows:: Went to my forum's admin dashboard, then to Support, then "I have a problem" - this told me the name of the MySQL table that had crashed (core_sessions), and I went and used PHP MyAdmin to repair it. After this the forum was fine and the dashboard advised I should run a few tasks that had failed to run while the database was unavailable. No guarantee this will work for others but adding it just in case anyone searches for the error.