Check to make sure wire connections on heater elements under grip are still connected, the soldered connections break pretty easy.. Use a meter to see if you have power there if not trace wires down from there..
unplug the hand warmer at the conection and see if there is continuity through the warmer itself. And as mentioned above check the wiring for broken wires. That would be the first step. No continuity means that there is a break in the element in the grip and it needs to be replaced.