Call Nook customer service 1-800-THE-BOOK (1-800-843-2665) and tell them the problem you are having. They will email you something and you go to a Barnes & Noble and you pick up a new Nook. I had that problem with mine and I had to get a new one. Make sure you can access your online B&N account so that you can put everything back on your new Nook.
Had same prob...Fixed it by holding on power button while hitting "n" button on bottom center of screen! good luck!