Make sure you use the correct username/email address and password combination when you try to log in.
If you continue to have problems, click on the Forgot your password link on the login page to reset your account.
Submit the email address you used to register the account. If you have several email addresses that you could have used, submit each in turn to identify the correct email address.
You will be sent an email to the correct address with instructions and a link you can use to reset your password. It may take several minutes for this email to arrive. You should check your email spam folder if it does not appear.