Wauw, this is the 100th time i have to tell someone how to redownload game files.
I don't think you can fix it. It would take a long time to find out which file doesn't work but you can redownload them if you just want to be able to play again.
- Delete the files you "messed around with"
- Go to Steam library
- Right click Black Ops MP
- Press "Properties"
- Go to the "Local files" tab
- Press "Verify integrity of game cache
It will check any files are missing and redownload them
Of course some of you pro BO modders may know another solution if you want to keep you changes...