The White Screen of Death (or WSOD) can be caused by a lot of different things. There is an article about them http://drupal.org/node/158043
Without more information, especially error messages or logs, all answers will be guesses.
Follow the steps in the article above, in order and one by one, to track down your problem. The first step is making sure you can see errors by adding:
to you index.php or settings.php. Once you do this, you should see what is happening and be able to address it.
That said, the two biggest culprits of WSOD are errors in custom code and hitting the PHP memory limit.