It looks like they attacked the first board.
code:
<span class="mainmenu"><a href="viewforum.php?f=1" class="mainmenu">Hacked</a></span>
It appears to be a problem with PHPBB2. You'll want to upgrade that every time there is an update.
I've seen the name PowerCobra before on a friends site that got hacked. I bet this is just the case of some script kiddy that wanted to be mean to you.
EDIT: Try your admin panel