Hasn't happened to me but have seen this before (I think two times). Not sure if there was any resolution.
One thing that I thought was interesting was a user saying it might be because your credit card had expired or something to that effect.
I actually returned my 1st Nook HD + because it kept doing this. Then the new replacement did the exact same thing, so I resigned myself to just having to randomly re-download books.
I even have a shelf called "Download" so that every time I found another "undownloaded" book in my library I put it there and then redownload them all at once.
I hate it. I hate not having access to any book in my library whenever I want, whether I have internet at that moment or not.
In a year or two I am going to get an iPad and put the Nook app on it instead of fighting with my Nook HD Plus. My old Nook Color (which I passed on to my mother) never had that issue.