Toolbox at the top, not vertically

[rich2005]

The short answer is use: Edit -> Preferences -> Interface -> Window Management and click on Reset Saved Window Positions to Default Values Then ok that, close Gimp and restart.

That gets the default 3 window mode back. For single window mode use Windows -> Single Window mode

A little longer with explanation why it might happen:

https://www.youtube.com/watch?v=ZQz0zCyVFJU duration 4 minutes.