The answer to 'tune-up not heating' is that a variety of issues could be causing your HVAC system to not heat properly after a tune-up. Common causes include a malfunctioning thermostat, clogged air filters, issues with the furnace itself, or problems with the ductwork.
If your system is not heating after a recent tune-up, first check the thermostat to ensure it is set properly and the batteries are working. Next, replace the air filters, as dirty or clogged filters can restrict airflow and prevent proper heating. If those simple fixes don't resolve the issue, it's likely a problem with the furnace or ductwork that will require a professional HVAC technician to diagnose and repair.