Ouch. I'm guessing you were hacking around at the model file, without really knowing what you were up to? I gather it's tedious stuff since it's not intended to be hand-edited really. CA generates it from a spreadsheet or a tool almost certainly. If you managed to trash something about the models, I'd think it would be crazy difficult to figure out what exactly was wrong...

If I've correctly guessed what happened, perhaps you'd share a bit more about the specifics of the experience for the benefit of those who may be trying to get their feet wet?