When you log-in on the iPad, there should be a password forgot option. Tap on it and follow the procedure to reset your password. Access the email you used to created the account and reset the password, then try logging in. Once you logged in, perform a factory reset to remove other issues that you might encounter or already have.
Additionally, go to Settings > iCloud and remove your account, then sign in again, but this also involves in having an Apple ID account working and ready to be accessed.