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.
I’ve looked online for days on how to fix this issue with my computer not recognizing that my Nook was plugged in. I did a hard reset like you said (powered off until I got a total white screen) and it worked right away! Unbelievable, I was on B&N forums and everything, no one mentioned this! Thanks so much!!!