I assume you verified
propane is available (stove lights)
Sometimes a wire on the rear of the switch comes unplugged and sometimes the connector to the control board gets unplugged by pulling the power cord out of the storage compartment.
Ours was giving us trouble, I replaced the control board with the Dinosaur Electronics board and it fires up every time.
There are several YouTube videos of step by step troubleshooting tips for the water heater.