I'm assuming that your device used to charge normally in the past. Try the things below.
Battery does not charge from the USB power adapter
If iPhone is connected to a computer that's turned off or is in sleep or standby mode, the battery may drain.
Make sure the outlet being used is working.
Try another USB power adapter if available.
Check out the 30-pin connector on the iPad and check the cable end as well. Sometimes dust gets lodged in there making a poor connection. You can use a soft brush along with trying to blow it out
iPhone: Hardware troubleshooting
If this doesn't work, I would take it to the closest Apple store or Authorized Service Center.