The answer to 'air handler not turning on' is that there can be several common causes, including a tripped circuit breaker, 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 you can try yourself include checking the circuit breaker, inspecting the air filter, and testing the thermostat. However, if the issue persists or you're unsure of the cause, it's best to call a professional HVAC technician to diagnose and repair the problem. They have the expertise and tools to properly identify and fix the issue.