Happened to me before on my Bold9000. Eventually it came back... Though to this date I'm not sure how... I would say the whole patients spiel right about now, but it could possibly be a bug.
Posted from my CrackBerry at wapforums.crackberry.com
I would check under options/applications to see if it is there. If so delete it there then go to www.blackberry.com/messenger and download it again fresh.