if it doesnot come up try to press and hold the power buttom and the home at the same time if you can go to settings then try to unregister register the nook if it doesnot work try to do a hard reset on the nookTo Hard Reset NOOK Tablet and NOOK Color
1. Hold the power button down until NOOK Color is powered off, ignoring the "Power off" prompt.
2. After Your NOOK is powered off hold the "NOOK Home Key" down then simultaneously hold down the "Power" button, release the buttons immediately after the “Read Forever” logo disappears.
3. If successful, a grey NOOK Logo will appear and the screen will refresh momentarily and flash yellow. A prompt will appear asking if you would like to perform a “Factory Reset,” Tap the "NOOK Home key" to confirm. Tap the "Home key" again to begin the process.
4. After a few minutes your device will reset and re-install all necessary software. Once completed, you will have to register the device again.