I've checked through your transactions and all of your recent orders, including the most recent order with no download attempts from the 16th, appear to be for a product that no longer exists in your account. If this was a special, time sensitive product that you removed from your account yesterday or today that would explain why the links are no longer valid and can't be reactivated.
Recreating the product won't help the original link, but you could still add it back to your system and use the Send Free Download Link option to make a new link to the new product for the buyer who didn't download in time.
