I am working on a notebook. I have somehow gotten it into a state where the usual top Mathematica line - the line that has pull-down menus "File", "Cell", etc, - is gone, and all I see is a window with the actual Mathematica numbered expressions and evaluations. That window fills the screen, can't be shrunk. At the top left of the screen, I see "Title" (pull down choices), Save icon, print icon, option inspector, some formatting icons, and "go back" which seems to do nothing.
I can't even exit from Mathematica in any normal way; I have to get out of this by Cntrl-Alt-Delete and use the Task Manager to kill the Mathematica task.
I don't know how i got into this state, but i'd like to get back to normal.
Solved this myself: F12 toggles back and forth from this display to the normal display.
On Mac OS Control+Command+F toggles full screen mode.