Activating debug in phpBB
Does your forum not work properly or you don't know what happened to it after you installed some phpBB mods or/and themes? Activate debugging for your phpBB forum. To enable it and see the errors you should edit the config.php file located in the root folder of your forum's installation and un-comment (means remove two forward slashes) the following line:
// @define('DEBUG', true);
// @define('DEBUG_EXTRA, true);
Unable to see the errors output after you enabled phpBB debug?
It is possible that the feature that displays PHP errors might have been disabled by your hosting provider. In that case you can enable error repoting directly in your .htaccess file