when i trying to launch the l4d2 after the
intro it suddenly shut off even thou i tried retstart the computer
plz solve the problem plz

When Valve got around to fixing the game after rolling out a broken update (both L4D2 and Steam had updates today, no clue which one broke the game), I was one of 20 people on the continent playing the game. It was like playing Soldier of Fortune 3 with six entire servers to choose from.

Valve assures me that the game is fixed now. I want to believe them.

Valve lied to me. They didn't fix the problem.

But I was able to roll out another fix a month back and since then L4D2 has been working flawlessly. (the same flaw also affected Supreme Commander 2 and Modern Warfare 2). It was related to their "steamcloud" service.