MyBB SQL Error

MyBB has experienced an internal SQL error and cannot continue.

SQL Error:
145 - Table './newforum/mybb_users' is marked as crashed and should be repaired
Query:
SELECT u.*, u.username AS userusername, p.* FROM mybb_posts p LEFT JOIN mybb_users u ON (u.uid=p.uid) WHERE p.tid='19' AND p.visible='1' ORDER BY p.dateline