I'm not sure if this happens on PC, but with every update of the MC Launcher and Minecraft since the new launcher, the fullscreen option has not worked.
HOW TO DUPLICATE:
Currently I use a Macbook Pro. I havent seen this bug anywhere else, but in the new launcher, whenever I click "Fullscreen: OFF", the entire screen on my computer turns pitch black. Yet the game is still there, because I remember where the buttons are on the screen, and they are still able to be interacted with. If you hit ESC and walk around, you can hear walking. Also, if you are in a server, you stay connected just fine. The game itself runs perfectly fine; it's just that your entire screen is covered in a black layer, basically. So the game isn't unresponsive.
HOW TO FIX:
Well if you are caught in this mode, there are two ways to fix it.
1. If you remember where the Fullscreen button is in a fullscreen mode, you can click it through the black screen and return back to normal.
2. If you do NOT remember where the button is, you can go into your minecraft folder in Application Support (.minecraft for PCs via %appdata%), go to the options.txt, and turn "fullscreen:true" to "fullscreen:false"
To test to see it wasn't just 1.6.2, I then did 1.5.2 in the new launcher and the same thing happened. Black screen.