Does any bulb work with that endcap? If so then you have a bad bulb, not an endcap. If nothing works in that endcap, then is it on a balast by itself? If it is paired with another endcap on the same balast and the other endcap works, then your balast is good but your wiring is bad. Check your wires in that endcap and make sure they are not loose or covered in a burned coating, preventing the electricity from flowing.