Most problems with heaters of all types can be summed up as "element failure".
It is often simply a case of metal fatigue. All that can be done here, is to replace the faulty item.
The other issue may be a faulty thermostat. The symptom here could well be the heater being too hot, or too cold (or even completely dead). Once again, replacement is the only expediant. Please return to the Home Page to find your make and model.
For further help with repairs, try www.howtomendit.com