vBulletin 5.1.0 crashes all of a sudden

[vibethemes]

Today morning I woke up and saw that my vBulletin forums have crashed. I am seeing this error :
Fatal error: Call to a member function fetch_userinfo() on a non-object in /home/XXX/public_html/forums/core/vb/library/user.php on line 721

I've checked the database and there are no crashed tables. Any reason why this could happen and how can I correct this ?
vbulletin 5.1.0

p.s: I've tried vbulletin forums but have got no support.

[Zachery]

That's odd, since I know I responded to your post.

[vibethemes]

Yup, I saw your response only after after I posted here.

The issue is now fixed.

FIXED : I checked my email messages and found that the Cache table was crashed, however in phpmyadmin it was not showing so. I repaired the table and everything now seems to be working correctly.

So next time if your forums stop working all of a sudden, repair the cache table.

Thanks for all your help !