If you can't get anyting else to work try locating your database and delete it. On Vista and Windows 7 its located in the hidden folder appdata your logon directory
\Users\LOGONID\AppData\Roaming\Barnes & Noble\DesktopReader file ClientAPI.db
I had an interesting experience today. I tried loading a book that I purchased in the B&N Libraray, and the book would not load. I called the 800 number and unregistered then re-registered the Nook. That didn't work, so I was told to archive, then unarchive, and that didn't work. What did work was for me to download the PUB file, then copy that file into the my documents section while the Nook was connected to the computer like a thumb drive. A similar thing happened to another book that I already had read (Wooden on Leadership), so I downloaded that title. Even though you copy the file to the my documents section, you still have to tap the check for new content option on the Nook. Be careful, if you click on the permanently delete hyperlink, Barnes and Noble will permanently delete AND expunge the title on the website, which also removes the title in the B&N library on your nook, AND remove the title in the PC version (the PC reader). If you are fortunate to download the PUB file BEFORE permanently deleting the title online, the ONLY option you have is to read the book in the my documents section of the Nook.
It is fairly obvious to me, since I work in an IT planning environment, that the version 1.2 of the operating system is still buggy, and the information technology division of Barnes and Noble (or whoever is performing this work if it's outsourced), needs to make a fairly DRASTIC maintenance enhancement release of the operating software so that instances of books not loading do not occur...EVER. Currently, there are WAY too many software defects resident in the current operating system that need to be removed so that the software can perform as desired, and the Nook users can enjoy their book reading and continue to read books on their Nooks WITHOUT having to TROUBLESHOOT and spend INORDINATE amounts of time performing work arounds. I was encouraged that I got through fairly quickly today, it being Easter Sunday, and got connected to an English speaking help desk technician who performed a quite competent level 2 help support. Obviously, the IT environment was very much aware of these problems and on purpose deployed help desk support KNOWING full well that all of the wrinkles and unexpected behavior had not been completely ironed out and removed.
Had I known before I purchased my Nook the inherent and latent software defects that exist in the Nook system (the one I have is using version 1.2 of the operating system), I honestly would have purchased an Amazon Kindle instead!!! I will hobble along and read the rest of the content that I have purchased, but it is VERY unlikely that I will purchase any additional titles until the maintenance release number has elevated to at least 2.0 or even 2.1. Once that maintenance level has been downloaded on my Nook, I will buy ONE and ONLY one title, and see if it loads. If I have to call the 800 number at that time because the title does not load, I will turn off the Nook, and put it and all of its accessories, to include the power cable/USB cable into a plastic freezer bag, and sell it twenty years from now on Ebay. By that time, it will probably be an antique, and I will probably get more money for it that I paid for it.