That's a very strange problem.
Are you charging from PC or wall plug. Some PCs, and particularly low power USB ports don't supply enough power to both charge and run Nook.
So you'll get Not Charging.
However, you can turn on Nook, plug it in and use. IT'll show Not charging but it'll be better than using without being plugged in.
Note: Plugged into PC and screen on. Won't charge. But battery will drain slower than if you were not plugged in.
Plugged into PC and screen off. Will charge, but slower than will wall plug.
Plugged into wall plug and screen off - fastest charging.
Plugged into wall plug and screen on - should both charge and be usable.
If you're already using a wall plug then don't know why you're getting this.