Forum mysql error - 2006 - MySQL server has gone away

[محروم.كوم]

I keep getting this error today, on the online.php page,
my host had me rebuild and optimise myphp database, but the error continues, off and on today.
They aren't helping. Any idea what this means, so I can get them to look into it?

I read up on this, but didn't find any solid answer I can go back to the host with.

Code:
Database error in vBulletin 4.0.1:
Invalid SQL:
SELECT thread.title, thread.prefixid, thread.threadid, thread.forumid, thread.postuserid, thread.visible
,post.pagetext AS preview
, NOT ISNULL(subscribethread.subscribethreadid) AS issubscribed
FROM thread AS thread
LEFT JOIN post AS post ON(post.postid = thread.firstpostid)
LEFT JOIN subscribethread AS subscribethread ON(subscribethread.threadid = thread.threadid AND subscribethread.userid = 1 AND canview = 1)
WHERE thread.threadid IN (0,561,567,506);
MySQL Error : MySQL server has gone away
Error Number : 2006
Request Date : Monday, April 19th 2010 @ 01:41:50 PM
MySQL Version : __DEFINE_LIKE_SHARE__