Check the filter, if dirty replace immediately, recommend every 3-6 months replace standard 1” or 2” filters. Most larger filters 6-12 months.
Check thermostat for correct setting. If digital display is out, open thermostat and replace battery.
Wherever the furnace or heating unit is located, there is usually an on/off switch nearby (looks just like a light switch). Sometimes this switch gets shut off. If furnace is not working, check to make sure this switch is in the “ON” position.
Gas smell — shut off the unit and call your local gas provider or AGL first.