Guided access

Force-restart your iPad:

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.

When you are stuck in Guided Access mode, you are really stuck as the typical triple tap on the home button to quit out of Guided Access doesn't work. Even a forced exit won't work because the power button and the home button are disabled. 

The only way to get out of Guided Access is to press the Home and Power button together for 15 seconds. This should turn off Guided Access by forcibly rebooting your device. Once your device is restarted, you can then go to Settings > Accessibility > Guided Access > Guided Access - and turn off Guided Access if needed.

Use Guided Access on iPad - Apple Support

Use Guided Access with iPhone, iPad, and iPod touch - Apple Support