Several factors can cause a furnace to not blow hot air. It could be due to a malfunctioning thermostat, a clogged air filter restricting airflow, a pilot light or ignition system issue, a faulty limit switch, or a malfunctioning heating element. Regular maintenance and professional inspections can help prevent and identify these issues. If your furnace is not blowing hot air, it's advisable to contact an HVAC technician for a thorough diagnosis and repair to ensure the system's safe and efficient operation.