I had a similar Laars Jandy when I moved in , few things to try:
1. Turn off and wait for a minute. Turn temp gauge to high on the exterior dial of the unit. Flick the rocker switch to ON, unit should fire up after a delay (can be up to 20 seconds).
2. If you hear the gas flow after a short delay but the heater doesn't light check the
a. Does the unit have a shut gas shut off valve either exterior or inside the unit. Mine had 2 for some reason. Trace your gas source and make sure they are on.
b. Make sure your pilot light is lit and properly adjusted to ignite the gas as it turns on.
3. turn off any booster pumps and repeat step one. There flow sensor on these units that can be fidgety, without enough water flowing through the burner the unit will either cycle or never turn on. Booster pumps can cause this.
4. make sure the fire service cut off switch is still in the unit.
*shudder* I dont miss my Laars unit.
Good luck!