The answer to 'why is my water heater not turning on' is that there are several common causes, including a tripped circuit breaker, faulty thermostat, or sediment buildup in the tank. The water heater may not be receiving power, the thermostat could be malfunctioning, or mineral deposits can prevent the heating elements from working properly.
If the water heater is not turning on, first check the circuit breaker to ensure it has not tripped. If the breaker is fine, then the issue may be with the thermostat or heating elements. Depending on the age and type of water heater, you may be able to troubleshoot and repair it yourself, but for older or more complex systems, it's best to call in a professional HVAC technician.