This symptom can indicate several possible issues: an internal hardware failure, a low/faulty battery, or just a simple memory/firmware glitch.
The first step here will be to attempt to reboot the nook. This is done by pressing/sliding and holding the power button for 30+ seconds, then release. Give the nook about a minute to start back up, and let me know if the problem persists, we can then try some further steps.
Note: I apologize for any duplication if you've done this or some variation of it before, but please do try to follow through these specific steps, as the timing is critical, and it can also take 4-5 tries before the reset process takes effect.
If it still isn’t working properly, the next step will be a deeper level reset. If you have not charged the nook recently or know that it is at least half full, then you should charge the battery for at least a few hours before proceeding further. Then plug the nook into computer via usb cable, then press and hold power button for 60+ seconds, and release. The nook will take 3-5 minutes to reload it's internal firmware. If it doesn't turn back on by itself, press power once to turn it on.
Let me know the results.