White Screen of Death

Enable Error Reporting

Although it may be turned off on commercial hosts and production sites (for good reason, so that users do not see the errors), these errors are one of your best tools for troubleshooting. To enable error reporting, temporarily edit your index.php file (normally located in your root directory) directly after the first opening PHP tag (do not edit the actual file info!) to add the following:

<?php

error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);

// $Id: index.php,v 1.94 2007/12/26...

You will now be able to see any errors that are occurring directly on the screen. Memory problems may still not be displayed, but it's the first step in a process of elimination.

Extract from: Blank pages or "white screen of death" (WSOD)