The answer to 'my replacement won't turn on' is that there are several common causes, including a tripped circuit breaker, wiring issues, or a problem with the thermostat or control board. First, check the circuit breaker to ensure it hasn't tripped. If the breaker is fine, inspect the wiring connections to make sure they are secure and not damaged. You should also check the thermostat to ensure it is properly programmed and communicating with the HVAC unit.
If these basic troubleshooting steps don't resolve the issue, it's likely that there is a more complex problem with the HVAC system that will require a professional HVAC technician to diagnose and repair. Attempting to service the system yourself without the proper training and tools can be dangerous and may cause further damage.