I tried all the previous recommended attempts and none worked for me.

I created the problem when editing the color of a line in a scatter plot. Suddenly I lost many (most) of the features of a multi-tab spreadsheet (in every tab)
* I tried saving as a 2003 spreadsheet --> the save worked, but no functionality was regained
* I tried grouping spreadsheets and then ungrouping them; no improvement
* None of the sheets were protected or shared.

Finally, I tried copying one of the tabs to a new (unsaved) spreadsheet. The tab in the new spreadsheet had all functionality!!

So I selected all the spreadsheets (this groups them), then copied the group to a different new spreadsheet (using "Move/Copy" with copy box toggled). In the new spreadsheet, all functionality was returned (amazing). I saved the new spreadsheet and regained nearly a week of work. Interestingly, simply making a copy of the flawed (corrupted?) Excel file does not accomplish any improvement. A simple file copy is as flawed as the original.

However I disabled the features (greyed out), I believe is due to a bug in the Excel software.