The answer to 'air handler not turning on' is that there could be several common causes, including a tripped circuit breaker, a faulty thermostat, or issues with the air handler itself. Symptoms may include no power to the air handler, the fan not running, or the unit not responding to the thermostat.
Some potential solutions include checking the circuit breaker, inspecting the thermostat for proper operation, and verifying that the air handler has power. However, if you are not comfortable troubleshooting electrical issues or accessing the air handler, it's best to call a professional HVAC technician to diagnose and repair the problem.