I have a Nikon SB-600 that was working just fine during a wedding and then stopped firing. I removed it from the off shoe cord in hopes that the cord might be damaged and attached it directly to the camera, but that did not work. I had a back up and just placed it to the side to finish out the wedding but now I am trying to see what is wrong with it. It will come on but will not fire/flash not even when I press the test button. It has never been dropped and I have older ones that were used more that still work. Is there a way to replace the strobe flash in the unit or could something else be wrong?