I'd think it's simply an over temp switch. basically cuts the 50 volts going to the driver transistors if they get shorted or driven super hot.
you have a little nubbin of wire there where it broke off, measure across it and the remaining wire with a ohmmeter and see if it's dead short (switch) or has resistance (thermistor)
if it's just a temp switch you can probably just jump the switch points with a wire to get going again until you can source another.
if indeed that's what it is, something like a "normally closed" 90c would be decent enough for this application. Just find a way to clamp it down.
if it is some kind of compensation thermistor, you will have to find a part number of it and find it on the schematic to find out what it's specs are.