I had issue with Nook HD going green for a few seconds then amber for a few seconds then nothing.
As stated above its because the battery went flat after sitting for too long w/o charger.

If you only have one charger you can try the plugging in, green to amber then you, un-plug, wait a second or two, replug and repeat cycle, or try another charger.
I tried the plug unplug for about 10 times then tried the wall-wart from a Samsung tablet and it stayed amber.
Lesson learned: keep the sucker charged up! Even new Samsung tablet will discharge over time if not kept charged.