What would cause my air handler not to come on?

When your indoor air handler fails to come on or blow air through your home's ductwork, it is usually experiencing an electrical power interruption, a tripped safety switch, or a failed blower motor component. First, check your main electrical breaker panel to ensure the circuit breaker dedicated to the air handler hasn't tripped. Second, inspect the 3-amp or 5-amp low-voltage control fuse on the indoor furnace control board, as a blown fuse will prevent control signals from reaching the relay. Third, check the emergency switch or safety float switch connected to your condensate drain pan; if the drain line is clogged, the safety switch will cut power to prevent water damage. You can troubleshoot by clearing clogs and replacing blown fuses. You can verify that power has been successfully restored when the blower motor starts running normally.