Most common cause is burn out of the thermal cut-out. These are a small screw on device on the exhaust duct, when exhaust air reaches too high a temperature, like a blockage, they fail before a fire.
Clean the exhaust ducts all the way to the outside wall, lots of vent hoods block over time. then examin the exhaust duct for devices that look like this or like this with wires connected to the terminals. there may be 2(hi lo), test with a flashlilight bulb or ohmmeter conductivity good, no conductivity replace.
there are only a few different kinds the local parts guy will have them all. take the dud one for him to match.