Perhaps it happened when you plugged in or detached an external monitor. Maybe you just hit the wrong key somehow, but now your Windows desktop is sideways, positioned in portrait mode while your screen is in landscape (horizontal) view. You can’t even read the text without tilting your head or turning your display on its side.
Hit CTRL + ALT + Up Arrow and your Windows desktop should return to landscape mode. You can rotate the screen to portrait or upside-down landscape, by hitting CTRL + ALT + Left Arrow, Right Arrow or Down arrow.
If, for some reason, the shortcut keys don’t work (and they should), try this method.