This may sound too simple, but it worked. I have an ipad 12.9 gen 3. It refused to start and would not charge. If I plugged in a charger, the screen would display a battery with one thin red line. No matter how longer I left the unit on the charger, there was no joy. The local Apple store declared it dead and wouldn’t touch it. Then I took it to a local UbreakIfix and they suggested replacing the battery, They discovered, however, parts for a third generation ipads were impossible to get. Yet, when I went to pick up the unit, they had it working. How? They used a 24 Watt charger (Samsung) to wake the battery up. The battery had been left to totally discharge a few times. Apparently, the charging circuit will not charge a fully depleted battery. This answer may not work for everyone, but it worked for me.