Well, someone pressed the autorotate button, that's what. Screen rotation displays content in landscape (horizontal) or portrait (vertical), and isn't available for all apps, but Samsung's One UI actually allows you to rotate the whole interface in landscape mode, too.
Galaxy S20 screen auto rotate for larger pictures and landscape interface mode:Here's how to make the
1. Pull down the status bar, and check if "Portrait" is written under a switch at the top row.
2. Tap on the Portrait toggle and change it to the much more self-explanatory Autorotate.
3. You can manually force the rotation each time be tapping on the small corner icon that appears when you turn the phone sideways, too.
For rotating the Galaxy S20 home screen from portrait to horizontal layout, however, you also need to go to Settings > Display > Home screen, and set Rotate to landscape.