There are two thermostats, sold as pair. One is a "one shot" and has a coloured dot on it (mine is a green dot) the other is a normal thermostat switch which switches on and off depending on the temperature.
They are at the back of the machine, under the silver cover which can be removed.
It sounds like the one-shot thermostat (the one with the coloured dot) is gone on your machine. Something obviously made it "go", so there is probably another fault - maybe the heater element or fluff caught somewhere reducing air flow.
If you let me know your email address, I'll send you photos to help locate them.