Possible causes include a malfunctioning thermostat, a clogged air filter restricting airflow, a faulty ignition system, or a malfunctioning gas valve. Additionally, issues with the blower motor or a malfunctioning limit switch could be contributing to the problem. It's advisable to consult a professional HVAC technician to diagnose and address the specific issue, as continuous operation without heat can lead to increased energy consumption and potential damage to the furnace.