I had the very same problem today. I had updated one of my plugins and all of the sudden all of the non-admin areas came up blank.
Eventually I found out how to fix it. It seems that WP had tried to revert to the default theme for some reason. Several months ago, I had deleted the default theme from the wp-contents folder to same a few bits. When WP reverted to the default, there was nothing for it to revert to.
So I reloaded the default theme folder from an old backup, everything loaded up! It was then just a matter of reactivating my custom theme and everything works now.
