I used to service the large hps lamps in parking lots and such,we called them salt lamps.Does the fixture have a seperate igniter or is built into the ballast? The igniter will be a small aluminum cylinder 2 or 3 inches tall about one inch in diameter,usually with a small hole in the top.With the power off flick it with your finger,take care not to dent it.Often inverting a fixture after it has been used a while makes the igniter unhappy.If the igniter is built in I'm not sure if tapping on the ballast will help or not.If the fixture is hard wired turn the wire nuts over and check for broken wires that way.sometimes solid wires will break when you reattach the wire nuts but the connection will still look good!hope this helps.