You might try re-uploading the files from the distribution. The only time I see that happen is when either:
1. something didn't upload and it is throwing an error you cannot see.
2. someone changed something (like fields on the tickets) and flubbed it.
Alternatively if you want to really troubleshoot the problem, you should check out the errors in your php.log. You can also turn errors on (directions below):
open up /main.inc.php and look at line 42
CHANGE
ini_set('display_errors',0);
ini_set('display_startup_errors',0);
TO
ini_set('display_errors',1);
ini_set('display_startup_errors',1);
That should make errors display on the screen and then you do not need to look at the php.log.
