I had the same issue that Marion and Terry had (3 years ago!) but I don't think the question was understood - when I select the WORK AREA button, the ribbon goes away & the spreadsheet window is maximized (correctly), but there is no floating 'work area' icon. Without that icon, there is nothing I can do to bring back the ribbon except quit Excel & re-start it.

** I have just discovered that 'work area' icon - it was floating on a second monitor!!! I didn't see it because it was lost in my other windows. I had to ALT-TAB to find it.
I would suggest to KuTools, that you add this icon to the navigation window that is defaulting to the left margin.

Also - when I restore the work area, my Excel maximizes in its monitor. I only want it to return to its original window size.
This may be particular to WIndows 10, or Excel 2016, or something else, so it could be a bug; but if everyone else has this happen, then it should be fixed in future versions - perhaps add a choice: 'return ribbon & window to original size' or 'return ribbon & maximize window'?