First try pressing and holding the power button for 20-seconds and then release. Try starting it up again.
If that doesn't work take out the battery and unplug the USB connection. Next, plug in the USB connection BUT immediately hold down all four page forward/back buttons (on top of nook) at once. The bottom touchpad should light up and start updating. This may help restore your nook.
For a more detailed discussion on handling a frozen nook screen checkout this forum thread.