Check the thermostat
Confirm heat mode and the temperature setting. If the thermostat uses replaceable batteries and the display is weak or blank, that is a reasonable first check.
Check power and airflow
Confirm the furnace has power, inspect an accessible filter, and make sure supply and return vents are open. Do not bypass switches or remove combustion components.
Know when to stop troubleshooting
If there is a gas odor, combustion concern, repeated breaker trip, or other safety symptom, stop and follow appropriate safety guidance. Otherwise, call when safe basic checks do not restore heat.
Safety note: If you smell gas, see smoke or fire, suspect carbon monoxide, or believe there is an immediate safety danger, leave the area as appropriate and contact the relevant emergency or utility service. Do not treat an active safety emergency as a routine HVAC appointment.