Dark display

i got some problem. i opened a game, and the game is not working. so i did an Alt-Tab, but it cant do. so i did all ways like Alt-F4, Ctrl-Alt-Del, and still cant. so i forced to force shutdown by taking the battery off. and when i try to start it, the display is dark. not black, but dark coz i still can see the desktop. i checked the brightness, and its full. what happened to my laptop?