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?
If you are confident it's the flash tube that's bad, and are experienced in electronics, you can purchase a flash tube online and replace it. But, you will need the right knowledge, tools, and experience, and this is not as simple as, say, simply unplugging something and replacing it. Desoldering and soldering are involved.
Safely discharging the capacitor in the flash is a must. The voltages are very high and can potentially be lethal. A service manual will help you a lot. OEM flashes typically have a contact point that you can use to short a resistor between it and the foot plate (ground) to safely discharge the capacitor. Using a voltmeter to see the voltage is low enough to be safe is probably a really good idea. Also, speedlight capacitors have been known to hold over 200V even with the batteries out for 24 hours, so don't think battery removal alone is sufficient.
If you are not someone who services their own car or messes about with their computer, or can't do the math to figure out how big a resistor you're going to need, this may not be the path for you. The following Youtube video demonstrates the steps involved in replacing the bulb.
(The "s" in fresnel is silent :-), but it's still a decent video).
Hopefully, that should give you an idea of what's involved and whether or not you feel confident in tackling the task, or it's completely worth it to pay a service center to do it for you.
All the testing that can be done has been, the ready light comes on - so it's charging ok, but with no output either through the commander mode or test button the guess at the flash tube is likely to be correct.
Replacing a flash tube can be done but is absolutely not a job for the inexperienced. Even when turned off and with the batteries removed some components in the flash can store a serious amount of energy; plenty enough to give you a very nasty shock and potentially enough to be fatal if not handled correctly.
Don't mess with it, take it to a service centre.