You already tried plugging it in and checking to see if it is charged, right?
If something was running in the background when it went "off", it should have run out of powerby now.
Assuming it has power, hold the power off button for 15 seconds, release for 4seconds, and then hit power button again(release right away, dont hold it.) See if that works.
Try what NookGardener wrote.
If that doesn't work
1) Plug it into the wall plug with the Nook cable.
Then leave it overnight.
2) The next morning try what NookGardener wrote.
To me it seems that the battery drained out. You'll have to leave it charging overnight. There's no Easy or Instant button.
Just thought I'd add that the battery is not user replaceable and not meant to be accessed. When it eventually wears out you have to contact customer service about sending it in for a new battery.