BuddyPress.org

The whole site is blank? Then try using the standard buddypress theme and see if its still blank. If it is, then try disabling buddypress and see what happens then, still blank, then its a wpmu problem, if not then its a buddypress problem and if the site works when using the default buddypress theme, then the theme your using doesnt work.

At least you’ll know where the problem lies with this.