I used Excel on a daily basis, but have never had to group rows, so I had to look up what you were talking about (and am I still not sure I understand its use).
Anyways- an easy solution (probably taking less time than trying to figure it out) may be to create a copy one of your good tabs (right click on the tab, and move and create a copy), and then paste data over from your bad tab.
Unfortunately, I couldn't do what you suggested, as there were literally hundreds of cells references to this tab from the other tabs in the workbook.
The solution had to do with making the grouping symbols visible. Had to dig, but ultimately found it.