How Do You Rotate Your Computer Screen?

I know it may seem like a silly question but, working on my laptop, my screen suddenly decided to change its orientation from horizontal to vertical so that I had to lie on my side to see the screen and type or turn the laptop on its side to see what was going on.

I have no idea how I achieved this, although I suspect it was something to do with the control key and an arrow from memory.  But, more importantly, how did I rectify the situation and restore my original settings so I could work efficiently.

How do you rotate your computer screen back to horizontal?

Fortunately Google had the answer in the form of  Microsoft Answers where there were two answers but the one that worked for me was  Ctrl + Alt + UP Arrow Key.