Your thermostat has to be set to HEAT and to a temperature a few degrees above the current room temp, or the system simply won't kick on. It's the most common 'no heat' cause of all.
Set it to HEAT (not COOL or OFF) and turn the target temperature up 3–5 degrees above the room. If the screen is blank, replace the batteries — many thermostats run on AA/AAA cells.
A clogged filter is the top cause of a system that runs but won't heat, and it can shut the unit down to protect itself. If the filter looks gray and packed, replace it (or vacuum a reusable one) and give it 30 minutes.
Find your furnace/air-handler breaker in the panel and push it fully OFF, then back ON. Also make sure any switch on or near the furnace (it looks like a light switch) is ON.
Furniture, rugs, or closed registers in the rooms you use can make the whole house feel cold. Open and unblock the supply and return vents.
If you have gas heat and other gas appliances (stove, water heater) are also out, the gas supply may be the issue — contact the gas company. Never try to relight anything if you smell gas.
You smell gas or rotten eggs, hear hissing, or feel unwell — leave the home and call the gas company or 911 first, then call us. Don't flip switches or relight anything.
Nice — warm again. If the heat struggles to keep up or cycles on and off oddly, put in a work order so we can service it before the next cold snap.
Let us know your thermostat type and what the screen shows — it helps us diagnose before we even arrive.