Please try the following in order (last two are courtesy MaluhiaKana)
Power Off: Press and Hold Power button for 20 seconds.
Power On: Press and Hold Power button for 4 seconds.
2) Restart Method 2
Power Off: Press and Hold Power button and while keeping it pressed press and hold physical 'n' button (the one below the screen) for 20 seconds.
Power On in case it doesn't start automatically: Press and hold Power button for 4 seconds.
3) THESE MIGHT WORK. Not sure.
Option 1: 8 interrupted boots in a row (http://nookdevs.com/Flash_back_to_clean_stock_ROM) will restore the original firmware. In my case: 1.1.0
Option 2: From a completely off-state, press and hold the power + N button for exactly 8 seconds, using your PC’s clock as a timer. Release both buttons at 8 seconds. It may take a few tries to get it right. Follow the prompts to reset your unit. But it keeps your current firmware version (1.2.0).
Q: How do you know it's fully charged?
Use the wall charger and leave it plugged in overnight if the methods I listed above don't work.
Shine a very bright flash light at an angle to the display surface; move your head around from the other side.
If you see hints of an actual display, then the back-light has failed (this happened on one of my digital camera's once -- cost ~$200 to replace the display module).
On my Zune's, if they are fully dead, it takes nearly 15 minutes on the charger before one can almost see the "dead battery"/charging icon <G>
The same thing happened to me yesterday it just webt blk. I took it to the closes b& N n they called the Co. Their answer was that they didnt know why n that it hasnt happened bfore ( obviously they were lying) n basically say "Sorry there is norhing we can do" so now im stock with an almost brand new nook dh color that cost almost $300 n doesnt work!!! Great customer service B & N. Im very dissapointed n never in my life will recommend anyone to buy a nook!! I heard apple has the best customer service n respond to their product quality way better! They even replace it with a new one. So guess where i am spending my money now??