Just press Ctrl + Alt + <Arrow Up>.
Sometimes you accidentally press some key combination on the keyboard and your screen turns upside down. If your computer is turned upside down, now you can turn it back. 🙂
Windows 8 combination Ctrl + Alt + <Arrow Up>  restore screen back to normal position. I encourage you to try other combinations like Ctrl + Alt + <Arrow Left> and so on and play around this feature.
Actually I don’t know what is the purpose of this feature. I can imagine to use it when computer is connected to ceiling mounted projector and then your picture have to be turned upside down. But usually ceiling mounted projector picture have been already turned upside down in its settings and there is no need to turn monitor on computer.
Anyway, I had a little bit harder problem. After turning my monitor to normal position for some reason it turned on a side automatically after I switched to some application. I pressed Ctrl + Alt + <Arrow Up> and got screen back in correct position, but screen turned on one of the sides again after I opened or switched to some other application.
I have to mention that I worked on a notebook when I experienced this strange behaviour.
Finally I found out that Windows 8 screen resolution settings have property “Allow the screen to auto-rotate”. When this attribute is checked notebook acts like smart phone adapting screen orientation to phone position. I found out that turning notebook on one or another side screen automatically adapts to position. I checked out mentioned property and problem with rotation was gone. Unfortunately I have to admit that my HP ProBook orientation sensor works poorly.
Time to time it is useful to review application shortcuts. Please, find here Windows 8 short cuts: http://windows.microsoft.com/en-us/windows-8/keyboard-shortcuts.
See pictures below.
P.S. Surprise, surprise – just found out that Windows logo key + O locks and unlocks screen auto rotation!
Documentation of hard work 