Check the controls you normally use
If there are no warning signs of a gas, CO or electrical emergency, look at the thermostat display. Confirm the selected mode is Heat and the requested temperature is above the displayed room temperature. Check whether a schedule or hold setting explains the change. For a battery-powered thermostat, follow its user instructions if the low-battery indicator appears.
Look for an obvious airflow obstruction
Make sure furniture and rugs are not blocking supply registers or return grilles. If your filter is accessible as part of normal owner maintenance, check it according to the equipment instructions. Do not operate the furnace without its filter, open combustion compartments or bypass a safety switch. These observations can help a technician, but they cannot diagnose an ignition or combustion fault.
Know when to stop
If heating does not resume, contact a qualified technician. Record the thermostat reading, when heat stopped and any error visible without removing a panel. Do not repeatedly reset the furnace or cycle its power to force it to run. If there is gas odour, a CO alarm, smoke or another immediate danger, leave and call emergency assistance from a safe location instead of waiting for an online appointment.