Tools - Options - Calculation
Make sure Automatic is selected.

"Formulas not responding to changed data" wrote:

All of a sudden, worksheets with formulas that I have been using forever are
not responding to changes in data. I checked formulas and formatting - all is
correct. The cell with the formula in it does not show a changed result when
the related data changes.....Seems suddenly to have happened to all my
spreadsheets!