I am not familiar with Excel on the Mac, so this is a guess:
Are the cells showing ###rather than xxx
In either case try expanding the column width. Sometimes values do not show if there is insufficient room in the cell to display them.
If this is not the problem, then please give some more detail.
Are these cells with xxx cells that previously displayed a number and now don't display the number, or did xxx appear when you tried to enter a number.
Is there any difference between the numbers that do display and those that don't - include examples.
If for example cell A1 contains xxx, what happens if you enter =A1/1000 in another cell.