Screen zooming may be enabled by accident, so fonts and icons become huge and the virtual screen doesn't fit in the display, which forces users to scroll in every direction all the time and makes it very hard to work.
If this happens, and you go to Display Settings, all you can see is up to four virtual monitors plugged in, usually 43" ones, but you can't get rid of this annoying effect.
This is an accessibility feature of Gnome, and it can be easily switched on and off with a combination of keys. The default keyboard shortcut to enable or disable this feature is Alt+Super+8.
So, in Windows press Alt+WindowsKey+8 simultaneously. In the Mac it's Option+Command+8.