You don't state which Nook device you have but this seems most common with the HD models so here is the recommended fix from the Nook forums. 

To Hard Reset NOOK HD/NOOK HD+, complete the following:
1. Confirm the device is powered off.
2. Hold the Power and Home key n simultaneously.
3. Release the Power and Home key once the NOOK logo appears. A Factory Reset prompt will appear.
4. To continue, press the Home key.
5. To exit, press the Power key.

Hope that helps. I will be posting this in the Nook Community Support Forums on the BandN site too.