Screen is blank after pressing something on keyboard

I pressed a button on my keyboard by mistake I think it was one of the buttons on the top of the keyboard, I was on youtube at the time and after I pressed this button the screen went blank but the music kept playing I pressed the spacebar and the music stopped and when I pressed it again it played again so the keyboard is working but the screen is blank. I turned the conputer off and on but the problem is still here someone help!! I have a windows 7 by the way