I can’t turn off my iPad mini

Generally, it is unnecessary (and for most users, unproductive) to fully shut-down an iPad when not being used - as background tasks will be running to download email, perform backup and other housekeeping tasks. However, if being set aside for long periods, you can shut-down the iPad if you choose to do so. See >>> Turn iPad on or off - Apple Support

Otherwise, If your iPad is unresponsive, connect your iPad to its Power Adapter and allow the iPad to charge. While still connected to external power, try a forced-restart:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

Note:  If iPad still doesn’t turn on, or if it gets stuck during start up, see the Apple Support article If your iPhone, iPad, or iPod touch won’t turn on or is frozen. Or if your iPad isn’t working correctly after you restart it, see the iPad Support website.

If you continue to experience difficulties, you will need to seek assistance directly from from Apple Support. You can contact the Support Team using the Get Support link at top-right of this page. Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:

https://apps.apple.com/gb/app/apple-support/id1130498044

You might also 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:

Genius Bar Reservation and Apple Support Options - Apple

Why not? What happens when you try?

Is it frozen? Is it non-responsive?

Are you following these steps: link-> Turn iPad on or off - Apple Support