I tried plugging in my Nook to re-charge and remove some PDFs I had completed, but I kept getting a message saying that the USB device was not recognized. I tried this from various USB ports (sometimes devices only want to use the rear ports on my computer), but that didn’t fix it. I even checked it on my Linux PC to see if it could find the Nook; no dice.
After checking out this very informative post on Nook basics, I gave the hard reboot a try, and now it works again. This fix wasn’t specified anywhere in the post, but there was a comment about a hard reboot (holding down the power button for 12-20 seconds) fixing software issues, and that’s apparently what this was.