Okay. Then I think you should turn on error reporting for a while, to see if there is anything informative.
You can do that by editing the wp-config.php file on your server, arount line 82 you should see this:
define('WP_DEBUG', false);
change it temporary to:
define('WP_DEBUG', true);
then let’s hope that some error messages will pop up, that would be the best.