You should consult the error logs. (PHP and webserver)
If there is nothing there you might try re-uploading the distribution files.
You can also turn on error displaying in OST by opening 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);
Then let us know which errors you are getting.