What do you mean "changed it?" Changed your account name or password? Presently, if you're unable to log into the game, there's no way to manually change game account names or passwords without the intervention of a GM or administrator. Also, what error are you getting when trying to login?
Do you mean your forum account name or password? Because those are entirely separate and not shared by the game login.
UPDATE: The new
password reset feature is now live. If you're still getting errors during login, try resetting the password using the e-mail address you registered your account with. Make sure it's less than 16 characters in length.