I have a 1993 challenger 200amp panel. This panel is in my shop and has 8 ft fluorescent light that are on 15 amp breakers. 3 sets per breaker. All the neutral wires have melted insulation on them 2 inches out from the Neutral buss bar. Could you please tell me why this would happen. Every thing on the other side bar is OK...

If it were only one wire with melted insulation I would suspect a loose connection; but if it's every wire on the bar, I have no idea. Have you had any unusual things going on electrically in your shop lately?

Someone else here will probably provide a better answer, in the mean time, I would shut off power to the panel.

... are you sharing neutrals on these circuits?
Are the ballasts all magnetic or electronic?
What size were the neutral conductors?

Aside from the above questions, I would strongly recommend measuring the neutral current on all the neutrals after you repair them... non-linear loads and poorly connected neutral sharing circuits can contribute to overload conditions on the neutral conductors...
Don't guess at this... put an amprobe on the neutrals and let us know!

I suspect only one wire was loose and the heat migrated to the other conductors.

Cut off the burnt portion of each conductor to get to clean copper.

You should replace this neutral bar, unless there are clean wire ports to use. Do not use the same holes the burnt wires were in.