I’ll share with you what I’ve found to be the most common cause, but first there’re a few things you can do to at least get back up and running again.
If you have Dashboard access, first disable ALL of your plugins. 9 times out of 10, you’ll see your site reappear.
If you still see just a white screen, the next step is to switch themes. Activate WordPress’s default theme (the twenty-ten theme, now) and check your site again.
If that didn’t fix your problem, you’ll need to start verifying core files and such, or employ the services of a WordPress professional to help you get your site back up and running.