Left 4 Dead 2

Hi kirid. Welcome to TSF.

Is there any error message that comes up when you are unable to access these games? Make sure that your firewall software and/or antivirus aren't blocking the game from accessing the network.

So does this only happen for L4D2? Not with any other games?

Try verifying L4D2's game cache. Go to the game's properties in Steam and select the tab "Local Files", then press the button "Verify integrity of game cache".

If that doesn't help, try uninstalling L4D2 with RevoUninstaller, restart your computer and then reinstall.