After reading the troubleshooting steps you already performed we, just users like you, cannot diagnose that kind of issue remotely.
The only thing I would do further is to put it on charge for at least half an hour, then, keeping it plugged in, try a "forced" restart of your iPad, using these gestures:
In rapid sequence, press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the power button. Only when the Apple logo appears (it might take 10-20 seconds), release that button (ignore all other screens that might appear, the one with the "slider" to turn the device off included).
If the issue still persists, contact Apple Support directly, using this link:--> Contact Apple for support and service
Regards
Giulio