Assuming that you mean when you are using the power adapter and it is plugged into a wall socket, have you tried using a different USB to dock connector cable? A different power adapter? Another wall socket?
Start with this and if it still won't charge, you should think about restoring the iOS software. Reboot the iPad.
Reboot the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider - let go of the buttons.
If it still will not charge, restore the iPad.If that doesn't work, you probably have a hardware problem. Take it to an Apple store for evaluation.