Yeah, you can rummage through the trash to try and find your case. D;
Don't throw away or lose CD keys for games that need them... you never know when you might need to reinstall or something.
If you haven't yet registered a Steam account with your CD key, and you can't find your CD key, then you're probably just best off
buying HL1 again and getting a fresh CD key (that you won't throw away this time!)