The CD key for "Counter-Strike: Condition Zero" is located on a document that was included in the original packaging. There is no way to recover this CD key if it is lost.
Although the CD-ROM version of "Counter-Strike: Condition Zero" is no longer sold by retailers, a digital copy of the game is still sold and distributed by Steam, which is a free gaming service. Visit Steam's official website to download its digital gaming software, then use the software to purchase the game. This version of the game works on Windows, Mac and Linux systems, with a person's Steam login credentials used to authenticate and play the game.