Can't launch game

[zong]

I'm using a mid-2012 macbook pro with a 2.6 ghz intel core i7 processor, nvidia geforce gt 650m and 16gb ram. I've played the game for a few hours and it was running fine, but now I can't seem to launch it. I keep getting an error box with the letter F in it whenever I start the game from steam. I've reinstalled it a couple of times and it's still not working. Any help would be appreciated thanks!