First make sure that you are using the correct username / email address and password combination when you are trying to login. If you continue to have problems use the Forgot your password link on the login page to reset your account password.
You will need to submit the email address you used to register the account. If you have several email addresses that you could have used you may need to submit each in turn. You will then be sent an email with instructions and a link you can use to reset your password. Please bear in mind it may take several minutes for this email to arrive and you should check your email spam folder if it does not appear.