(1) the gas is cut off; (2)the pilot light blew out and is not igniting the burner; (3)the thermostat is set too low for your requirements; (3)the thermocoupler is bad; (4)the pressure relief valve is stuck in the open position and discharging the hot water. 1,2,& 3 are self-explanatory. The thermocoupler is a bullet-shaped sensor located near the burner - it senses the temperature and turns the gas on or off to the burner according to the setting of the thermostat. The pressure relief valve is usually located at the top of the water heater on the hot water line and leads to the floor and under the house (separate from regular service hot water line) - it opens when the temperature of the water is too hot and discharges the hot water to keep the heater tank from over-heating and exploding. Hope this helps. Answer one is very good, but there is one error which requires correction. The thermocouple does NOT, as described in answer one, act as a demand thermostat for turning the main burner gas ON and OFF. The demand thermostat probe extends from the back of the gas control valve INTO the water INSIDE the heater tank. On the other hand, the thermocouple is located so as to be IN THE PILOT FLAME. Its purpose is to determine the presense of the pilot flame, and if the pilot flame goes out, the thermocouple will cool down, at which time, it TURNS OFF the gas TO THE PILOT. This is a safety feature designed to prevent the continued release of gas, which which if gaining contact with an open flame or any spark, would result in a devastating explosion.j3h.