Did you plug it into your computer or to a wall socket?
Which Nook do you have and how long have you had it?
Did the green light ever change to another color?
Have you tried another cable?
My daughters did the same thing (NOOK color) - had a 12 month warranty and this happened in the 13th month, go figure...Took it to local B&N store and they called support - nothing they could do since the warranty was expired + they do not make the NOOK color anymore so did not even have the option to replace it - bottom line I am screwed. My guess is the battery - which they cannot replace as they cannot open the device to get to it - going to buy a Kindle Fire tomorrow and leave B&N forever and WILL tell everyone about this sad excuse for customer service...