How do I force shut off my iPad?

Here you go.

If your screen is black or frozen

If your screen is black or frozen, you might need to force restart your device. A force restart won't erase the content on your device. You can force restart your device even if the screen is black or the buttons aren't responding. Follow these steps:

• On an iPhone X, iPhone 8, or iPhone 8 Plus: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the Apple logo.
• On an iPhone 7 or iPhone 7 Plus: Press and hold both the Side and Volume Down buttons for at least 10 seconds, until you see the Apple logo.
• On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Home and the Top (or Side) buttons for at least 10 seconds, until you see the Apple logo.

See the following from If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support

On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Home and the Top (or Side) buttons for at least 10 seconds, until you see the Apple logo.

Why do you want to force shut it off? You can reset it, or you can turn it off. Do you have a problem with one of the buttons? The sleep/wake button is how you power it off. Holding down the sleep/wake and home buttons together is how you reset it.

Try the reset. Have you had any other issues with the touchscreen in the past?

See my previous response, as well as the one from FoxFifth. Both describe a reset.

On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Home and the Top (or Side) buttons for at least 10 seconds, until you see the Apple logo.

You're welcome.