My screen turned sideways, how do I rotate it back?

Is it a 2-in-1 or a laptop that folds back? Then it might be auto-rotate rather than a shortcut, it flips the screen when it thinks you've turned it. Open quick settings (Win + A) and turn on Rotation lock once it's the right way up.

Right-click the desktop > Display settings > Display orientation, set it back to Landscape and click Keep changes. Navigating with a sideways mouse is the annoying part, go slow. You probably hit Ctrl + Alt + an arrow key by accident. Some Intel graphics drivers use that as a rotate shortcut, so Ctrl + Alt + Up may flip it straight back, but plenty of newer drivers have it switched off so don't count on it.