Oh, I tried holding down the power button, no dice.
Sounds like what happened to mine. I took it to the brick and mortar B&N and had the salesperson at the nook desk look at it. I hadn't had it a full year so it was still under warranty. They called the office or headquarters or whatever and got the ok to replace it right there. Very convienient.
Sounds stupid, but some people with that problem used most original decision- cleaned their NST screens, including "near-sensor" areas. Can't harm anyway.. Well, unless you're careful enough)
This happened to mine last night. I've had it less than two months.
Nothing has worked: I've tried holding down a variety of keys for crazy amounts of time, tried charging, tried cleaning. Nothing is working!
This is very frustrating and it looks like a very common problem.