Apple Users

If it's not booting while connected to power, your problem may be something other than a bad battery!

I'd consider ...

(1) Make sure it's actually connected to power. Assume for now that the cable is bad, try a different cable between the USB adapter and the iPad and wait an hour or two and see if it comes up.

(2) It might need a force-reset. Instructions from https://www.digitaltrends.com/mobile/how-to-reset-an-ipad/

If you have any iPad with a Home button, such as the iPad Air, then here’s how to force restart your iPad:

Hold down the Power button and the Home button together until the iPad turns off and reboots. You can let go when you see the Apple logo on the screen.
If you have an iPad without a Home button, such as the 2018 iPad Pro, you need to:

Press and quickly release the Volume Up button.
Press and quickly release the Volume Down button.
Then press and hold the Power button until the device restarts.