The answer to 'my water heater won't turn on' is that there are several common causes, including a tripped circuit breaker, faulty heating element, thermostat issues, or sediment buildup. First, check the circuit breaker and reset it if tripped. If that doesn't work, you may need to replace a faulty heating element or thermostat. Sediment buildup can also prevent the heater from turning on, so flushing the tank may help.
If you're not comfortable troubleshooting and repairing the water heater yourself, it's best to call a professional HVAC technician. Attempting DIY repairs on a water heater can be dangerous if you don't have the right skills and equipment. A qualified technician can diagnose the problem and make the necessary repairs safely.