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get some kind of MySQL frontend (example using phpmyadmin, but use whatever) ... open your database, find and choose the yourprefix_sessions table, (you should see it listed up top now) click operations, click repair table.
might as well check all the tables that start with yourprefix_bb_ while you're at it... just choose the table (on the left) & if you see red in the space usage block on the right side repair that table also.
Wheeeeee.....