Regaining Access to Areas After Losing Saved Games

http://support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/std_adp.php?p_faqid=373

If you have deleted your saved games, or have lost them for any other reason, you can do the following to regain access to the later chapters of the game.

First open the console (if this is not enabled you will need to do so by going to Options, going to the Keyboard tab, then Advanced, then placing a check next to Enable the Developers console), type in 'SV_Cheats 1' without the quotes and press enter, now type in 'SV_Unlockedchapters 15' without the quotes and press enter. You can now select New game from the main menu and choose the last chapter you played.