Option 1. Hard restart.
Press the Top Volume button
- Press and quickly release the volume up button.
- Press and quickly release the volume down button.
- Press and hold the top Power button until you see the apple logo appear



Refer to YouTube video guide https://www.youtube.com/watch?v=nu6U2zcyUYQ
Option 2. FS/Smartalock App restart
This option would only work if Guided Access is disabled and FS/Smartalock app frozen but the iPad itself is still responding.
FrozenKioskScreen.mp4