Are you using the charger that came with the iPad? If so there are only a few things that can be wrong. The best way to charge an iPad is to use the charger that came with it.
1. Software problem in the iPad. First thing to try is to reset your device. Press and hold the Home and Sleep buttons simultaneously until the Apple logo appears. Let go of the buttons and let the device restart. See if that fixes your problem.
2. No power in the outlet. Check the outlet to make sure it has power.
3. Bad cable. Try a different cable.
4. Bad charger. Try a different charger.
5. Dirt or fuzz in the connection port. Check and carefully clean out any debris.
6. Hardware failure in the iPad. Make an appointment at an Apple Store to have your device examined by a technician. Or contact Apple Support.