The problem may be the USB cable, but before you spend $25 let's try:
1) power down your Nook and then plug it in. this should turn it back on and charge it.
2) Soemtimes my NC cord light would stay green until i jiggled it a bit.
When you use the cable to connect to your PC, does the PC see the NC? If not, this is a good sign that the cable is bad.
I had the same problem with a newly purchased nook color. took it to the nearest BN store and they gave me a new usb cable. no problems since. worth a try.
I have to 'mickey mouse' my cable into my nook everytime I charge it. I have to wiggle it and slightly push down on the cord to sort of make the inside tilt upward.
I have found that if I run my cord into the nightstand table from the wall, the extra cord into the drawer and my nook on the edge of the table top, closing the drawer so the cord is pulled down, that I don't have to babysit the thing to ensure it stays on orange to charge. If I don't do this, I have to keep a close eye on it so that it doesn't turn to red.
I am still using my original cord because I refuse to pay for a new one that may or may not fix the problem. Once this nook dies (and I think it's close to it - It won't hold a charge for more than 2 days when 98% of that time is when the nook is turned off among other things) I'm going to check out other options either a Kindle Fire/Tablet or an Ipad.