The answer to 'my thermostat won't turn on' is that there are several common causes, including a dead battery, faulty wiring, or a problem with the HVAC system itself. First, check the batteries in the thermostat and replace them if needed. If the batteries are fine, the issue may be with the thermostat's wiring or connections. Try resetting the thermostat by turning off the power at the circuit breaker, waiting a few minutes, and then turning the power back on.
If the thermostat still won't turn on, the problem may be with your HVAC system. This could be due to a tripped circuit breaker, a faulty control board, or other mechanical issues. In this case, it's best to call a professional HVAC technician to diagnose and repair the problem.