There are a few possible reasons. First, make sure your membership hasn’t expired yet. It’s also possible that you are using the wrong username/password combination. Check your purchase confirmation e-mail that was sent to you when you originally signed up. All of your login info will be in that e-mail.
Check your junk/spam folder if you are using a web-based email service such as Hotmail, Yahoo or Gmail, since they sometimes mistakingly send our e-mails there.
If you deleted the e-mail or you are still having trouble logging in, please contact our customer service center.