The answer to 'my air handler won't turn on' is that there are several common causes, including a tripped circuit breaker, faulty thermostat, clogged air filter, or issues with the air handler unit itself. First, check the circuit breaker to see if it has tripped and needs to be reset. Next, ensure the thermostat is set properly and functioning correctly. If those initial checks don't resolve the issue, the air filter may be clogged and restricting airflow, so replace it. If the problem persists, there could be an internal problem with the air handler that will require a professional HVAC technician to diagnose and repair.
If you've checked the basic troubleshooting steps and the air handler still won't turn on, it's best to call a licensed HVAC technician to inspect the system and determine the root cause. Attempting to repair the air handler yourself without proper training and equipment could be unsafe and lead to further damage.