There is only an on and off setting. No pilot. I'm assuming it has an electric starter. Again assuming, is there an amount of time to wait? How old is this unit? It's atop my parent's house.

It's a early 80s model. It will have electronic spark ignition. Its hard to say why its not working with out some trouble shooting. Open limit switch, bad spark control, bad gas valve the list goes on and on. With this old of a system you really need to let some one look at it to make sure its safe.