The two outside levers on your panel control the heat. At the bottom of travel, there's a microswitch that cuts off or enables heat. It's easy to push those levers in so they won't go all the way to the bottom, and that will keep the heat going, all the time. You should feel it latch into position when the levers are at the bottom and the heat is off.
It's also possible someone misadjusted those microswitches, or bypassed them, because they stopped working. To fix that, you'd have to have the control panel off.
