Hey guys, I am trying to launch CS:S and it says ''This game is currently unavailable, please try again later'' what should I do?

Restart steam

....oh shit, it might be the rumored Achivement Update.  O_O

" Restart steam "

I can get in easily enough.

I've had this happen before on other games, usually the solution involves Deleting Local Content and re-installing it. Or if you're lucky, Verify Integrity of Game Cache files might fix it.

Right click on the game - select Properties then Files and finally "Verify integrity of game cache"
 
That should fix it.

" Right click on the game - select Properties then Files and finally "Verify integrity of game cache"  That should fix it. "