I would recommend clearing your temporary files first. To do this, go to Tools, then Internet Options. Then click on the Delete Files button (be sure to mark "delete offline contents"). You can also clear the cookies but be warned, this will usually delete any type of webpage personalization you have at sites and may also clear out your user id/passwords.

If it is a paysite, make sure that the payment method you gave them is still good (credit card didnt expire etc). Other than that, email the site and see if thier support can see why you are unable to access the site.