Ok what you need to do is pull the dryer out and take off the buldge cover on the back.
From there you will see the heater unit. On there are 2 thermostats. Have a look at the bottom thermostat and see if theres a red button in the middle. If so press it in and if theres a click from it then the thermostat has just reset.
If no red button then you will need a multimeter to test it. If its faulty then the heater unit will need to be replaced.
Check it out and see what you find. If you need anymore info just get back to me.