power button not responding

You can turn-off (i.e., shut down) your iPad from settings - or using the Top (Sleep/Wake) button...

From iPad settings:

Settings > General > Shut Down

Shuting down using the Top button is equally straightforward. Simply press-and-hold the Top button until the shut-down slider appears on the iPad screen. When you see the slider, release the Top button, and slide with your finger.

Additional information can be found here:

Turn iPad on or off - Apple Support

If instead your iPad is unresponsive, you'll need to 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:

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.
4. 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.

See here: Turn iPad on or off - Apple Support

On some iPad models, you need to press more than 1 button in order to turn off the iPad.

~Lyssa