Help! Where did my cars go?

So, I was just tooling around, as I usually do, when I went to check up on one of my new models I'd created just tonight. Only when I got there, I received this strange error:

And then I discovered that most of the models I'd made....had simply vanished off the cars list(strangely, though, all my engines still seem to be there). And restarting the game hasn't helped.....I think it may have started, possibly, when I tried a fit an engine that was too big into another model(which is itself no longer accessible, unfortunately), but I'm not sure(Edit: btw, I tried deleting that engine; it didn't do a thing!).
Can somebody please tell me what to do and how this can be fixed? I really don't want to start all over again.....

And then I discovered that most of the models I'd made....had simply vanished off the cars list(strangely, though, all my engines still seem to be there). And restarting the game hasn't helped.....I think it may have started, possibly, when I tried a fit an engine that was too big into another model(which is itself no longer accessible, unfortunately), but I'm not sure(Edit: btw, I tried deleting that engine; it didn't do a thing!).
Can somebody please tell me what to do and how this can be fixed? I really don't want to start all over again.....
