Hello:
On some computers I have used, the display management software allowed the
screen image to be rotated. This could be done using either the display
management program in the system tray or with a hot-key combination used by
this program.
You may have just accidentally hit one of the hotkeys to rotate the display.
It can be tricky to use the computer once the screen is oriented from left
to right rather than from top to bottom, but it is manageable. Look for
options to rotate the screen in the display management software.
HTH