Do you have a white screen when starting the game ?

Okay, this issue is common and easy to fix. In fact, you have to download the "Portal: Prelude 2013 hotfix/patch (1.1.5)" from the Portal: Prelude download page here : http://www.portalprelude.com/download.php Then you have to follow the instructions. This is fast. You have to extract the "gameinfo.txt" in the portal prelude folder. This will replace the ancient one.