I've had problems with "turn off" button, as well. It's easier and more certain to just keep the power button depressed until it shuts off. Ignore the "Shut Off" pop-up.
On my NookC the "Power off" popup option never worked. I have to hold in the power button for a bit to actually shut down.
Another thing to remember - When you plug in NookC to charge it turns the unit back on. Sometimes new owners don't know this will happen.
Thanks for the replies, it worked.