Admin login stopped working

Can't login to admin anymore. Page seem to loop or not to do anything. No error message nothing. Here is what I got from the apache error log.

PHP Fatal error: Class 'queryFactory' not found in /var/www/vhosts/domain.com/httpdocs/store/includes/functions/sessions.php on line 54

One or more files on your site is damaged or missing.

If you've been uploading addons or making changes to files on your site, I suggest you carefully review what you've done and back out of things until it's working again.

If you've NOT been changing anything on your site, and your hosting company hasn't been doing anything to it, then maybe you've been hacked and should take appropriate recovery steps, including but not limited to applying the recent security patch:

http://www.zen-cart.com/forum/showthread.php?t=130161
http://www.zen-cart.com/wiki/index.php/Recovering_From_Hacks
http://www.zen-cart.com/forum/showthread.php?t=131115