Windows F11 key

Purchased the game and downloaded it. Waited for it to open and started exploring all the buttons. I acidentily hit the F11 key and now I can not any game graphics to display. we have deleted the game totally off the system and re downloaded it, waited for it to load and still cannot get game graphics to display to play the game only the opening music or sound plays on the pc speakers. Have to go to Task Manager to stop the game. The launcher window shows a grey block next to the Mojang logo not a red block when game was first started after download..