Database error in vBulletin 3.6.8: Please help

[alexchairman]

sir i am trying to post reply i am getting that type of error message. please help

Invalid SQL:

		SELECT posthash.threadid
		FROM posthash AS posthash
		WHERE posthash.userid = 3 AND
			posthash.dupehash = '88bb026bab9e26b4919bdc70b9f7362c' AND
			posthash.dateline > 1194325513;

MySQL Error  : Table './alexforum/posthash' is marked as crashed and should be repaired
Error Number : 145
Date         : Tuesday, November 6th 2007 @ 05:10:14 AM
Script       : http://xxxxxxxxxxxxx.com/forums/newreply.php?do=postreply&t=1955
Referrer     : http://xxxxxxxxxxxx.com/forums/newreply.php?do=newreply&noquote=1&p=3257
IP Address   : xxx.xxx.228.203
Username     : xxxxxxxxxxx
Classname    : vB_Database