It is frustrating.
You need to try to keep track of what you may have added or altered especially in relationto the mdlnames.txt file.
Have you set up more than one CFS2 install? that will help you--you can use one install to text out things you may be unsure about.
I also try to keep a log of all my add-ons, so that if there is aproblem I have some chane of recovery!
I just fixed up my own program--I had tried to make a copy of a ship file and rename it so that I could modify it while keeping the original--I or the program screwed up, but I just got it back.
The end of Cody Coyotes MB tutorial has a great list of the most important error messages and how to fix them.
If you tell us what you did last, I'm sure someone can help you.
good luck