It it saying wrong user/password?
This seems to be a problem for a lot of people.
Make sure your password has no symbols in it. If it has numbers, do not use the Num Lock pad to type them. Try making a pass with only letters.
Still doesn't work: reinstall the game.
This may not help, as I've encountered many posts about people unable to log on in-game. I'm afraid that if this possible glitch doesn't get fixed, you'll have to use your old account.