Did you try the password reset tool/link below the login box? Or did someone change the email address on your admin account as well? If your email has been changed too that's a pretty serious privacy breech.
I would get on the phone with GoDaddy support immediately, verify your identity and ask their assistance in changing your password. Then change all other passwords associated with your account and that site as well.
The other way to change the password is to go in through phpmyadmin in your GoDaddy dashboard and change directly in the database.