No telling what it could be. Have you had the fish long? What size tank is he in, and what other fish are in the tank? What are your ammonia, nitrite, and nitrate readings? Please post answers to these questions so we can help you figure this out.
In the poll, you asked "disease" or "just sick" but, like, aren't those the same thing??
I think sometimes high Nitrate levels causes headstanding, but who knows. Please post the info requested by Gunnie.