First, use a voltage regulator to determine if the problem is coming from the circuitry or resistors or the heating coil. Once the problem area has been pinpointed, replace the damaged components, and test the heater again to see if it is functioning correctly.
Portable electric heaters use a heating coil in order to generate heat, which is distributed with a fan. If the coil is not heating, the problem stems from a break in the circuits that activate the coil itself. Due to the steps involved in activating the coil, components such as circuits or resistors may be damaged and not properly running current to the coil. If the coil is damaged, it must be replaced entirely. In modern heaters, the coil is replaceable. Be sure to follow all the proper electrical procedures before replacing the coil to avoid an electrical shock.
In older models of heater, the coil may not be built for maintenance. In these models, a clamp is needed in order to complete the circuit and allow the coil to heat up properly. Ultimately, the coil is a piece of metal that used electrical resistance in order to heat up, which is what produces the heat the fan distributes, so the electrical systems must all be functioning correctly for the coil to heat up.