There was a sucurity flaw in the phpBB2 software that allowed someone to break into the server over the weekend and replace a lot of pages with a "You've been hacked" page. Fortunatley that's all they did. The bug has been fixed and everything should be back to normal now.
If you know any other sites running phpBB2 please inform the administrator to update to the latest version of phpBB2 or make the change outlined here: http://www.phpbb.com/phpBB/viewtopic.php?f=14&t=240513.