Device is unresponsive
First make sure that the iPad has power. Unplug it and reinsert the cable.
Does the screen show a big battery warning icon?
iPad ran out of battery. Make sure the wall plug, the charger and the cable are all working.
Does the screen show the lock screen but cannot be unlocked in any way?
App could not be installed or crashed. Kiosk lockdown needs to be removed. Contact firstname.lastname@example.org.
Did Support ask you to restart the device?
In order to force restart an iPad, please hold down Home button + Top button at the same time. See this Apple guide: Force restart iPad - Apple Support.
You may need to force restart several times before the iPad is released from kiosk lockdown.
Once the iPad is unlocked, contact Support to let them know.
App is not running
Find the app on the Home screen and start it. Once inside the app, press the Home button 3 times to activate Guided Access mode to lock the iPad into kiosk mode. See this Apple Guide: Use accessibility shortcuts on iPad - Apple Support
App refuse to run
Do you get a message similar to this?
You will need to enable Developer Mode. Open the Settings app then go to Privacy & Security -> Developer Mode. Now enable Developer Mode toggle and select "Restart". After restart a prompt should appear asking if you want to turn on Developer Mode. Press "Turn On".
Here is what it looks like: