Start outside the cabinet
Verify thermostat mode and setpoint, check whether the furnace has electrical power, and inspect the filter if accessible.
Do not bypass safety controls
Furnace safety switches exist for a reason. If the system will not complete its startup sequence, bypassing controls is not a safe diagnostic shortcut.
Call for diagnosis
If the furnace remains off after safe basic checks, or repeatedly attempts to start and stops, schedule service.
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.