update any modules lately?
Suggestion - only if you get desperate:
Get a full backup of the site, added modules, themes, etc.
Backup your database.
Take a breath and make sure you have everything.
Remove all non-core modules and run update.php.
Problem go away? At least you know it's software and not server.
Less brutal approach: Lookup disabling but not removing one module at a time, but of course without having to pull up a page to do it. I've seen this discussed a number of times at Drupal.org but don't recall where.
Interested in your results.