Windows probably told you that an update was pending and you then did something like clicking on 'Update and shut down' at the end of a session. That can be all that's needed for Windows to transition from 10 to 11.
As it shouldn't happen unless your computer is good enough to run Windows 11, it shouldn't really matter that much. Windows 11 looks a little different but it largely functions in much the same way as Windows 10 anyway.
Assuming that you have now got Windows 11, the most obvious change is probably that the Start button is no longer in the bottom right-hand corner but a bit closer to the centre of the taskbar down there. (Once you've clicked on it, the Power button is at the bottom right of the panel that opens).