In My Screen is Sideways, I explained how, using the mouse, you can fix a screen that has inexplicitly turned sideways. The problem, however, is that it's very difficult to use a mouse when the cursor movements onscreen don't match those of your hands.
The original question came from a forum discussion, and the discussion continued after I had posted the tip. And in that discussion, TonyRony offered a better solution.
You don't have to use a mouse to reorient the screen. Just press CTRL-ALT-UPARROW and your screen will reorient itself correctly. You can also use CTRL-ALT-LEFTARROW, -DOWNARROW, and -RIGHTARROW to orient the screen in other directions.
My thanks to Tony for offering this better solution.