Hmm... This is very interesting... Have you installed SMFShop previously? Do you have any other mods installed?
I've never seen anything like this happen with SMFShop before. Even if you get errors, your forum should still be working. Check to see if Index.php is still there, and if not, restore it from a backup.
Otherwise, try and restore from your latest backup. If you don't make backups, your webhost should have a daily backup available.
You can chat to me on MSN if you like (if you have MSN).