For no apparent reason the screen has moved round 90 degrees clockwise, anybody any suggestions, other than to lay on my side to use computer ?

If its an nVidia graphics card(which I'm guessing it is), head to your Control Panel, and open "Nvidia Control Panel" from within it.

In the "Display" tree there is a "rotate display" section.

This is probably where the problem lies. Short of that, its possibly your eyesight which has in fact changed by 90 degrees and you're just blaming your monitor!

Youve probably happened upon a keyboard shortcut for display rotation regardless of what card you have.

Find your graphics card options and set it back and then disable the rotation option.

Try Ctrl Alt and up arrow