It is possible that the server you are connecting to has split from the network, and if your account is new, it may not have received your new account information.

A few suggestions on passwords:

• The password you have chosen may have been too simple (like "god," "money," or a word related to your account name, like Name: Billy, Password: Bob), and somone may have guessed your password and changed it. We do not retain records of older passwords, and thus we are unable to verify previous ownership of accounts. We cannot change passwords that have been altered by another user.
• We suggest you use passwords containing a string of both numbers and letters six or more characters long.
• Never tell anyone your password. Blizzard Entertainment representatives will never request your password information.