I'm having a rather odd problem with my Excel formulas and hope someone can
help me.

Within the week, my Excel spreadsheet formulas have simply stopped working.
When I input new data into the sheet, the formula will not calculate new
totals like it did last week. Nothing about the formula has changed and it is
correct. Even if I alter a figure, it will not recalculate it just sits there
showing the same figure that it was before.

This has affected all my spreadsheets.

Has it been caused by an update? If so, how can I rectify it?