The answer to 'why is my water heater not turning on' is that there could be several common causes, such as a tripped circuit breaker, a faulty thermostat, or a problem with the heating element or other internal components. Symptoms may include no hot water, the tank not heating up, or the pilot light being out.
First, check the circuit breaker or fuse for the water heater and reset it if it has tripped. If that doesn't resolve the issue, the thermostat may need to be adjusted or replaced if it's not properly regulating the temperature. Other potential problems could be a broken heating element, sediment buildup, or issues with the gas control valve or pilot light on a gas water heater.