The most common cause of no hot water from your gas hot water system is the pilot light going out. Many newer systems have a piezoelectric starter which will light the pilot light for you. However, older systems require manual lighting with a flame.
The pilot light can go out because of a gas valve or supply, but the most likely cause is your thermocouple. Alternatively, you might have air in the gas line or low gas pressure.
If your thermocouple or flame sensor is faulty, you will likely need these parts replaced. This component senses the presence of a flame, and in its absence will prevent the gas valve from delivering gas to the water heater. If the gas control valve is faulty, this is an even more complex replacement.
Finally, if the gas burner is cracked or damaged, this usually requires a whole water heater replacement.
