Make sure that the USB cable is plugged securely into the power end. Have you tried a reboot of your Nook and charging then? If none of those work, then your charger is probably defective. Either call customer service or try taking it into your local B&N.
When my nook color is not plugged in, the battery indicator - both in the bottom right hand corner and under settings - reads as thought it IS charging. It literally says, "charging," even while the percentage of the charge that the battery carries is dropping! I am really confused why this is happening. Restarting has no effect. The NC is essentially brand new, and I manually updated to software version 1.2.0. Would really appreciate some technical assistance!
Restarting is not rebooting. Wake the Nook up, slide the green tab to the right. Once you're on the reading screen, or library or whatever screen....hold in the power button until it asks if you want to power the Nook off.
Wait about 3 to 5 minutes, until it is fully powered off. Now, hold in the power button until the screen flashes. It will fully reboot and the charging issue should go away. If it doesn't go away, contact the store where you got the NC (if within the first 14 days) to exchange it. If beyond the 14 days, call customer service and have them replace your NC.
I have noticed this a few times recently. I corrected it by plugging the charger in and then removing it as soon as the power level indicator comes up. I'm not sure that is the best thing to do, but it did make it change back to "discharging."