The thermostat should always be checked when the heater won’t turn on. Examine your circuit board to ensure it is generating power, and make sure the power is turned on. After that, again, ensure that the thermostat door is shut.
Additionally, if the door isn’t properly closed, a switch inside the door might turn off electricity to the appliance. If your thermostat is battery-powered, all you need to do is swap out the batteries. Make sure the plugs are not burned out if your appliance has them.