Make sure you are using the same email and password you set up during registration and payment.
This will be the email on which you’ve received the registration and/or payment confirmation.
If you originally logged in via the Facebook, Google, or Twitter login buttons, make sure to use them now as well.
Also, make sure you are logging in on the same website where you made your purchase.
If you are sure you are using the correct email and are on the right page, try resetting your password. See Password reset. for details.