Can't tell if this issue resolved or not, but you're right, it's not common.
Recently there seem to be intermittent problems accessing the shop at all, so it may be that you're able to access it again because something on the BN side started working again?
There was someone who was seeing an issue with a cellular / wifi hotspot (something like a mifi) who was able to use the internet, but not the shop. My take on that was that the cellular vendor was doing something unusual with a proxy that broke shop access, but I don't know that it was ever truly resolved.
One thing I'd be curious to hear, if you are still having the problem, is what happens if you install the Nook app on a computer and register it to the same email address your nook color is using- is the computer-based program able to search the shop? I tend to guess that it would be, and if so, if you buy a book using it, are you able to download the book to your PC? Does your NC hear about the book being in your library and present it for download? If so, is the NC actually able to download the book?
As a workaround, if the computer can download books but the NC still can't, you can copy the book file from your computer to the NC. Books that are registered to the same account can be read on both the NC and computer, and the files can just be copied straight across. On a Windows box, the files on the computer are stored in your
Documents\My Barnes & Noble eBooks\email address\