I don't know why that would happen and I'm not an expert on hosting stuff, but assuming you're desperate...I might try copying index.php to another name and see if that works, and maybe replace index.php with something simple just to make sure it's something to do with the code executing and not the server or something else. And if it does seem to be the code, assuming you have a current copy, you could upload all of the code again. (of course, all of this could turn out to be a waste of time

)
ETA: looks like redtailboa is restoring all his code...