there's a small capsule sewn into it about an 1" long and .250" in diameter that controls the switching from off to one under certain temps. this goes bad and the warmer wont heat up. replace it and you'll be good.

You can remove it completely and just twist the wires together, but make sure you keep an eye on the temp, because it won't be able to shut itself off and will eventually go into to meltdown. works in a crisis just make sure you don't leave them on for more than 40 mins or so.