Odd.
There isn't some driver software needed to make ADE recognise the Nook?
I know with some Sony devices (eg PRS-350, and some others) , the device will show up as an external drive without any special driver, but needs a special driver to be recognized by ADE. That it, if the driver is missing you get exactly symptoms as you describe. In the case of the Sony, the driver is installed (and uninstalled) together with the Soney Reader software.
I don't know if Nooks are similar; or if they just work directly with ADE and no special driver. Our Bookeen worked without a driver, which is much more convenient.
It might be worth checking if there is any similar driver needed for the Nook that has somehow got lost on your machine.
~~~
Meanwhile, as long as the Nook is still registered with your Adobe ID, you can copy DRM .epub books from the PC/Mac to the Nook using a file manager (Windows Explorer/Finder) and read them on the Nook. You will still need ADE in the picture to convert borrowed.bought .acsm files into DRM .epub files.
Of course, it the books are not DRM, you can just copy them anyway and never go near ADE. Either manage the library manually or use software such as the free Calibre. ADE has no benefits except for DRM books.