If the game requires a cd key and you still have it, it is legal since you are technically buying the cd key when you get the game. If it didnt have one, then it is not legal to torrent a game, but it is legal to create a backup of a copy you already own.
If you "lost" your copy, too bad. Get a replacement CD from the game manufacturer and take better care of it this time. Most game manufacturers will exchange a game disk for a new one for a nominal fee. It is still against the law for the person sharing the file to share it, and illegal for you to download the copyrighted material. Whether you own or owned a copy, or not, is irrelevant to the law regarding copyrighted materials.