If you have a gas water heater, common reasons for no hot water include the pilot light is out, the gas supply is turned off or, in a worse-case scenario, there is a gas leak. For electric water heaters, the cause may be a tripped breaker, a faulty high-temperature cutoff switch, or a flooded compartment that caused water to damage the thermostat. Faulty heating elements can cause your hot water heater to break down. It’s also possible the unit may be leaking.