Remove the sht metal box surrounding the burner tube then vacuum or blow out the rust flakes on top of the burner . If it still does not light remove the burner and clean the rust particles that have settled inside the burner tube and clean the orifice at the same time. While the burner tube is out check the slots to see if they are rusted out, if so the burner will have to be replaced. Just removing the rust flakes from the slots will probably get you refer working again but if you have not cleaned it before it would be a good idea. Does it light for a few secs then go out or does not light at all?