POS terminal running on Microsoft Windows have a neat feature baked into the operating system that allows you to rotate the screen using a keyboard shortcut.
If you find that your screen is sideways or upside down, it's most likely that the keyboard shortcut has been pressed by accident. (CTRL + ALT + Left, Right or Down keys will rotate the display sideways or upside down)
To rotate the screen back to normal, it's as easy as pressing the key combination of:
[CTRL] + [ALT] + [Up Arrow]
This will return the screen to it's normal upright orientation.