I was in the middle of shopping for a new book when my nook turned off. The power was still good and it was plugged in! It then turned back on but started to try to update and got about halfway through before it shut off again. It just keeps repeating this process. I am in a wifi area (at my house) so connecting to internet for the update should not be the problem.
That sounds like an update failed. If it fails to boot 8 times, the device should reload the original firmware to put it back in "like new" out of the box condition. If that fails, I'd try visiting a local B&N for assistance.
The good news is that the NC is a very tough little device. It should be recoverable if the unit is not physically damaged.