Check the thermostat mode, setpoint, and batteries first. Then look at the furnace switch and breaker, but do not reset a breaker repeatedly. A clogged filter can restrict airflow enough to trip a limit switch, so replace it with the correct size if it is visibly loaded. If the furnace still will not start, the likely causes move into ignition, flame sensing, pressure proving, controls, or fuel supply. Those checks need a professional. Leave immediately if you smell gas, see smoke, or a carbon-monoxide alarm sounds. During a cold snap, protect exposed plumbing and ask for emergency HVAC help rather than repeatedly cycling the equipment.
What to ask next
The right next step depends on the system, the urgency, and the building. Keep the model number, symptoms, age, and any previous service notes nearby when you request an estimate. A professional should explain the diagnosis or sizing basis in plain language.
When to bring in a professional
Use the callback form to describe the issue, or review HVAC services to find the closest starting point. For urgent loss of heat, see what to do in a cold snap.