The answer to 'my water heater won't turn on' is that there could be several common causes, including a tripped circuit breaker, a faulty thermostat, sediment buildup, or a problem with the heating element. The symptoms you may notice include no hot water, the water not getting hot enough, or the heater not turning on at all.
To troubleshoot, first check the circuit breaker and reset it if tripped. If that doesn't work, check the thermostat and make sure it's set to the proper temperature. You can also try draining the tank to remove any sediment buildup. If those DIY steps don't resolve the issue, it's best to call a professional HVAC technician to diagnose and repair the problem.