the volume button of my ipad doesn't work

Presumably you refer to the physical volume buttons on the edge of the iPad. If your iPad is within a fitted case, remove the case from the iPad - as some poorly designed cases can interfere with the physical buttons.

If you continue to have difficulties, try a forced-restart of your iPad - as this will often resolve an isolated software issue:

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.

If the problem persists, you may have a hardware fault requiring repair. 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 can explore your available options:

Genius Bar Reservation and Apple Support Options - Apple