"I acidently did by acident today" *snickers*
OK ok, this should be an easy enough fix... Do you have an nVidia crd? If so, rightclick on the desktop, click proporties. Go to settings, then advanced. You should have a tab with the name of your card(Mine says 6800GT) Click that then you should see NVRotate somewhere within the menus.
If that doesn't work, I feel quite sure it's in those settings(advanced settings) somewhere. Some computers even have the option on the taskbar(by the clock)
Good luck, and be more careful next time ;P