there website is down and has been for a while if someone can tell them i know how to fix the error
download navicat mysql

login your db and find db f34r1 then right click mybb_sessions go to manage tables and repair and it should fix the problem i used to work on sql mysqli seems different it most likely will work but mysqli is different so it might not

MySQLi error: 145
Table './f34r1/mybb_sessions' is marked as crashed and should be repaired
Query: SELECT * FROM mybb_sessions WHERE sid='0' AND ip='' LIMIT 1