Recently I've been experiencing this problem and could only fix it by playing in offline mode but as CS:S is an online game I can't do that in Offline Mode.
When I start the game it tries to launch but than gives an error of: This game is currently unavailable. please try again at another time.
This is already the same for over a month! Though it launches without a problem in offline mode!
I've already removed my whole steam folder except for steam.exe and steamapps and re downloaded everything, verified my cache and defraged the game data!
Though non of the above have done anything to solve this problem!
Is there any fix for this ?
Thanks for the help in advance, regards Rafaël De Jongh
Update: It seems it's not only with CS:S but with every other source game as well!