Somehow, the settings on your graphics card has changed. This setting is specific to your video card manufacturer, so it may be necessary to contact them to find out where this setting is located. But if you would like to prowl around, it shouldn't be that hard to find. I may even be able to get you pretty close. Here is where mine is located:
Go to Start>> Control Panel>> Display icon>> Settings tab>> Advanced button>> Intel(R) Extreme Graphics tab (or whatever your video card is)>> Graphics Properties button>> Rotation tab.
Or, possibly quicker:
1. Right click a blank area of your Desktop
2. Select Graphics Options
3. Choose Graphics properties
4. Select the Rotation tab
Good luck!