The forum is temporarily broken. It seems to be a problem on my host's side, and I contacted them about it yesterday.
Hopefully this problem won't last long.
Apparently there was a critical phpBB security flaw, and my host decided to disable all customer phpBB installations until they're updated. In the rush, they forgot to tell us.
I'll be upgrading the board tonight. Because I've hacked the code so much myself, it might take a little while before it looks right again.