What to do if iPad won't hard reset?

"Hard reset" does not appear in Apple's lexicon of terms for iPad. Presumably you actually refer to a forced-restart - and not a reset - as these are entirely different. Performing a forced-restart can be tricky, as the button sequence and timing is critical...

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:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button. Do not release the button until...
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

1. Press and hold the top button and the Home button at the same time.
2. Continue to hold both buttons; only when the Apple logo appears, release the buttons.

Assuming that the physical buttons are fully functional/undamaged, when performed correctly, the iPad will restart. If the iPad will not restart, you would be best advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP):

Genius Bar Reservation and Apple Support Options - Apple

iPad Reset

By contrast, a reset is performed from iPad settings. You'll not be able to perform a reset unless you can access the iPad settings page:

Settings > General > Transfer or Reset iPad >