If your iPad is completely unresponsive, connect your iPad to its Power Adapter and perform 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.

To fully shut-down your iPad using the Top (Sleep/Wake) button, press-and-hold the button until the slider appears on the screen - and then release the button. If instead you simply press and release the button, you will trigger Siri.

If you are having difficulty, instead the iPad can be simply shut-down from iPad settings:

Settings > General > Shut Down