A dryer that doesn't heat is sometimes due to a faulty high-limit thermostat. Manufacturers install these devices in both gas and electric dryers to protect the unit from overheating, but they require replacing if activated. Most other heat problems depend on the type of energy the dryer uses.
Gas dryers use an electric igniter to start the flame. If the igniter burns out, a replacement needs to be installed. Other potential problem parts include the valves that control the gas flow and a thermocouple that shuts off the gas if it doesn't sense the appropriate amount of heat from the burner.
Electric dryers sometimes stop heating due to a failed resistance heater coil. According to Easy Appliance Parts, these coils cannot be repaired and require replacement when they fail. The dryer timer circuit or a bad thermostat may not signal the element to start heating, so the drum turns without heating or drying the clothes.
If the dryer is heating, but the clothes remain wet at the end of the cycle, it's possible the exhaust vent has a clog. Lint from the clothes collects in the ducts blocking moisture from escaping. The high humidity doesn't allow any more evaporation in the drum. If a dryer requires more than one cycle to dry a load of clothes, the duct is often the cause.