HELP MY GAME IS UNPLAYABLE

So i was in a game and my music stopped so i put the game in windowed mode to open firefox and turn on my music. the window is now just a black screen and whenever i enter a game i just get a black window. i think its a problem with my PC because i get a similar issue with rocket league. but until i fix my Pc how do i put the game back in fullscreen from the menu because as far as i know you have to be in game to access the video setting. plz help