Computer screen only showing on top left corner?

So my cousin was playing the game WoW and told me it was only showing on the top left corner when he started it. I tried to fix it and when I minimized WoW, the whole computer screen became that way. So I kept rotating the screens with Ctrl Alt and the arrow keys to see if the resolution is the problem. I switched users and now the whole windows is stuck without any buttons to reverse it back to normal. Please I need help.