It could be a bad battery. or maybe a bad power adapter?
see here also
A few things:
1) are you using the power adapter and cable that came with your nook? -if not you should use the cable that came with your nook.
2) Its possible your charger/cable is bad. Try using a standard microusb cable attached to your computer (NOT the power adapter), you can still charge your nook, just more slowly and through your computer only (charge it while it is sleeping, not while using it).
3) When you say your nook gets to 100% charge, is the orange n on the cable turning green? if not, keep charging until it does turn green.
4) Can you use it while plugged in? or does it also become unresponsive?
5) You can try a factory reset: turn off the nook, hold down the power button and N button until a dialogue comes up asking if you want to reset.. you will lose all of your settings and content (which can be reloaded - assuming its from b&n or you have backups on your computer).
Recently while reading my nook powered off as if it had run out of battery when it was at about 50% power. It will not turn on since, even when holding the power button for 20 seconds and then trying again. At first it was unresponsive when I plugged the power in, but now it will charge to 100% and then over the course of a half hour will again become unresponsive. It if I leave the nook off, it will also not power back on after a half hour. So it seems like the problem is in the battery pack...it just doesn't seem to hold its charge.
I am currently outside of the states, and am hoping there may be some self fix it solutions so that I don't have to ship it back. Any help is greatly appreciated!!!! Thank you!!!