if 1shado1's fix doesnt work you can try the window button + P key combination to tell it to duplicate your screen :D, there are a couple different options when you press windows + P so you may need to push it a few times until you hit the correct one
are you sure your extending or duplicating the screen, try the Windows button + P key combination set it to either extend or duplicate and it should work, or your cord randomly crapped out on you :D its been known to happen.