The most common problem is a burner tube full of spider webs, mud-dauber nests, or dead stink bugs. Clean that thing out and if that doesn't do it, the most common part to fail is the thermocouple that ensures that the burner lights and stays lit while the gas valve is open. It is a cheap, easily replaced, and generic part. Last one I replaced cost like $5 at Lowes. Before you give up and take it to the tech who will charge you big bucks, it is worth just replacing that thermocouple out of hand.