Have a look at this link http://steamcommunity.com/app/72850/discussions/0/810920325460240676/ it seems to be a recent problem. The consensus is that it is a Steam problem which they need to fix as it seems to affect all Steam games on your system. I can confirm this as I have the same problem. I have had this problem for the last couple of days. Yesterday it went away for a few hours, came back again, went away and is now back again. A solution that works for me is to exit Steam via the task bar and restart Steam. Unfortunately you have to do this every time you exit your game, but it will allow you to play.