I also have a similar problem. I logged out of gamecenter somehow-I didn't even press the log out button. When I tried logging back in, it says my ID email is invalid, so i tried finding the ID I originally had on the apple website. When I entered the details, it says there's no account under the email.
Then, i made another account with another email to see if it still exists by searching my username, Raaahhh2011, and it's still there. Please help! I might lose my progress on my dragonvale and all, I really don't want to start again. I have also already tried what you guys said worked, but I have found no success.