Check your CC on your B&N nook account to make sure it is active and doesn't need to be updated.
I am having this problem!
I bought my nook three weeks ago. Wi-fi connected, easy. Account connected, easy. Bought a book, downloaded some samples...Now, nothing. Everything I search "0 Results" can't see my new book on my nook but it shows up on My Nook section of B&N site. :-(