Hi, I'm Nathan.Sorry to hear that you're having trouble, but I'll do my best to help you. Please give me a few moments to review your details here.
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.