1. Verify that the charging cable that you are using works correctly. Try it on a different device.
2. Ensure that you are using the iPad wall adapter and a charging cable that is not more than 3 feet (1 meter) long. The longer cables have too much voltage drop to provide a proper charge.
3. Let it charge I disturbed for at least 10 minutes before you try to power it on.
If none of that resolves your issue, then you either have a bad battery or a damaged charging port.