This Bug causes Portal 2 to just disappear and return to the desktop. This bug is now fixed, all you need to do is to upgrade your game. Google Portal 2 update. and download the First update and install it Your Game should work fine now.
There is also a series of other updates (update 2,3,4,5) you may get these as well to upgrade to the latest version, but for me just the first fix stopped the game from crashing to desktop.