How do I restart my iPad mini?
Try doing a "forced restart."

https://support.apple.com/en-us/HT201559

"You should force restart your iPhone, iPad, or iPod touch as a last resort, and only if it's not responding. To force restart your device, press and hold both the Sleep/Wake and Home buttons for at least ten seconds, until you see the Apple logo." 

When you see the Apple logo, release both buttons.