If you only see an on & off position on the gas valve controls, you don't have a standing pilot to manually relight.
Try turning off the power to the unit for a couple of minutes. Then turn it back on and turn up the thermostat. If it is an electronic ignition system it will probably relight itself and fire up. You would normally hear a clicking sound and possibly see the sparking of the igniter at the pilot assy. The pilot should light, the clicking sounds will slow down and within the time that the pilot system electrically confirms an established pilot, the main burners should come on.
Stand in front of the furnace and tell us what unfolds.