sciruss,
If its only the case you've lost and not the game manual, you're in luck.
The Key is in the back of your manual, and you should be away laughing again.
If you've lost everything but the CD, then I'm afraid you're up the proverbial creek without the proverbial paddle.
At this point we would normally point you to the
official forums or to send an email to BIS to see if there's any way of verifying your ownership of the game CD and to sort something out there.
Give it a shot, but given previous experience with people asking about/for CD-Keys, I wouldn't expect too warm a response.
Hope this helps slightly.