When a gas water heater fails to heat water, the issue may stem from various factors. Gas supply problems, such as a closed supply valve or a disconnected gas line, can prevent the burner from igniting. Faulty components like the thermocouple, gas valve, or igniter can also hinder ignition. Additionally, sediment buildup in the tank or heat exchanger can obstruct heat transfer, leading to insufficient heating. Addressing these issues requires troubleshooting to identify the root cause and take appropriate repair measures.