Keyboard or trackpad does not respond

Reseat the iPad: Take the iPad out of the case and put it back so it sits firmly.
Clean the contacts: Wipe the Smart Connector contacts on the iPad and on the case with a dry, soft cloth.
Compatibility: Does the case fit your iPad? You find the model under Settings › General › About.
Restart and update: Restart the iPad and update to the latest iPadOS.
Bluetooth keyboards: Charge the keyboard, remove it in the Bluetooth settings and pair it again. (Smart Connector keyboards need no pairing.)