You should be able to unlock the eBook from Barnes & Noble.com with this information:
Unlock Name: Full name on your Billing Address
Unlock Code: 15 or 16 digit credit card on record as your default credit card when you downloaded the eBook. Please replace the asterisks **** with the actual credit card numbers.
If you don't have the credit card you previously used to unlock the eBook, please try adding a new credit card on your Account page. After adding a new card, re-download your purchased eBooks, and try unlocking them with the new credit card number.
Note: Your NOOK will need to be connected to WiFi for this procedure to work properly.