Open the case, there's a thermistor in the middle of the heating plate, red, the one with the white silicon paste on it. Unplug and plug its connection socket several times to try to boot, eliminate poor contact, or not directly to buy a to change, about 2 dollars.
There is also a thermistor on the side of the heat sink, there is a big three-legged tube (IGBT tube) mounted on the heat sink, and on the top of the tube is a thing that is also coated with silicon paste, that is the thing. You need tools to replace that.