If you can prove your purchase (via a bank statement or receipt), the best course of action is to contact or Steam Support . They are usually very helpful in recovering licenses for legitimate owners.
Once you have purchased your key, follow these steps to download and install the game: Open Steam : Launch the Steam client and log into your account. Add a Game : Click on the menu at the top or the "+ ADD A GAME" button in the bottom-left corner of the library. Activate a Product "Activate a Product on Steam..."