As described, if the iPad has been stored unused and uncharged for an extended period, the battery may be completely dead.
Connect your iPad to its Power Adapter and allow the iPad to charge, undisturbed, for 24hrs. Then, while still connected to external power, try a forced-restart:
Force restart iPad (models with Face ID or with Touch ID in the top button)
If your iPad doesn’t have the Home button, do the following:
- Press and quickly release the volume button nearest to the top button.
- Press and quickly release the volume button farthest from the top button.
- Press and hold the top button.
- When the Apple logo appears, release the top button.
Force restart iPad (models with the Home button)
Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
If iPad still doesn’t turn on, the battery is almost certainly beyond recovery.
You would be best advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad - and you will be able to explore your available options:
Genius Bar Reservation and Apple Support Options - Apple