The igniter may be bad or the sensor that tells the gas valve it is hot so it is okay to turn on the gas. Depending on the brand, most I've seen use a glowing element rather than a spark. In a dark room, you can often see the glow of this when it gets hot (think stove or toaster electric elements that glow hot when activated). SOmetimes, if you're lucky, they have a schematic inside of the back cover. If you are really lucky, they still have the manual, which would have a parts list, theory of operation, and a diagram.
If it attempts to light, but the gas line is clogged (unusual event), no fuel, no burn. I think it is more likely it is not seeing the igniter, and therefore not opening the gas valve.