Safety first: if you smell gas, smell burning, or your carbon monoxide alarm is sounding, skip this list. Leave the house and call 911 or your gas company from outside.
1. Thermostat. Make sure it's set to Heat, the set temperature is above the room temperature, and the batteries aren't dead.
2. Breaker. Check that the breaker for your furnace or air handler hasn't tripped in the electrical panel.
3. Furnace power switch. There's usually a light-switch-style switch on or near the furnace. Make sure it's on.
4. Air filter. A very dirty filter can make the furnace overheat and shut itself off. Replacing it can help.
5. Vents and returns. Make sure supply vents and return grilles aren't blocked by furniture or rugs.
Still no heat? Call us at (703) 555-0142. After hours, our assistant will take your details and, if it's an emergency, connect you with the on-call technician.