What version of vB and what version of PHP? Has anything changed on the site or server since just before the problem occurred? Is there anything in your error_logs (if you don't know where they are, ask your host)?
4.2.2 pl1, PHP 5.4, I had made no changes and nothing had been changed for several days before this was found.
I have the error log and not knowing what I'm looking at does me no good. One item in the log that sticks out the most is this :
"AYAH: ayah_config.php missing; will require setting of specific values."
What the heck is ayah ?
If needed I can give you access to everything if you should feel so charitable