Some Left 4 Dead players have experienced crash errors since the release of today’s update which added a lobby browser. This error causes a “left4dead.exe has stopped working” or “left4dead.exe has encountered an error and must close” message to appear and then the game closes. In most cases, deleting and reinstalling the game will fix the error. However, there is a much easier method which won’t require you to re-download the game.
Right-click on the Left 4 Dead entry in your Steam games list and click Properties. Click onto the “Local files” tab and hit the “Verify integrity of game cache…” button. It will take a few minutes to verify game files and afterwards it may begin downloading data for a number of settings.
After you have done this, you should be able to play Left 4 Dead without fault. Six people I personally know have encountered this issue and this solved it; a number of other players on the Steam Community forums seem to have the same issue too. If you have a moment, leave a comment and tell us whether this did or did not help you!