I don't understand what you've done, so I'm making a guess.
Your NOOK knows your B&N account by its email address. Do not change your account's email address while your NOOK is registered to it.
If you've done that, change your account's email back temporarily. Then unregister your NOOK. It's in Settings>Device.
Once your NOOK is unregistered, then you can change the account's email address. Then reregister your NOOK to the account using the new email address.
If you need to check if you've got something registered to a B&N account, log into that account on your computer and go here: https://cart4.barnesandnoble.com/account/request.aspx?x=&stage=manageDevices but do not unregister from that page. That 'deregister' link is only for when you've lost your NOOK and you want to kill it from being able to rack up charges on your credit card.
When you re-register, your most recent B&N e-book purchases will be automatically downloaded to your NOOK. For older ones, you'll have to tell the NOOK to download them.
By the way, this is the forum for the B&N Reader software for iPads, iPhones, PCs, Macs, etc. There are separate forums for NOOKs.