The answer to 'water heater not turning on' is that there are several common causes, including a tripped circuit breaker, a malfunctioning thermostat, or a faulty heating element. Symptoms can include no hot water, the water not getting hot enough, or the water heater not turning on at all.
To troubleshoot, first check the circuit breaker and reset it if it has tripped. If that doesn't work, inspect the thermostat to ensure it's set properly and functioning. If the thermostat seems fine, the issue may be with the heating element, which will require replacement. While some basic troubleshooting can be done DIY, it's generally best to call a professional HVAC technician to diagnose and repair a water heater that is not turning on.