My Excel has freaked out.

All my excel files open up into some strange default view setting where only
the tool bar across the top is displayed. The actual sheet window is
hidden.

I figured out how to make it display the data sheet window by selcting View
FULL SCREEN from the tool bar once, then VIEW FULL SCREEN a 2nd time.

Where do I reset this behavior so the window opens properly to fullscreen
view ?

This is not a ingle file/sheet issue. It happens on any excel file open .

thanks for your help