Excel formatted totals

I have been using Excel for years, and the last few months I have had to click on the cell I want the total to update in.  It will not recalculate on its own.  Is there a good reason for this?  Some of my spreadsheets are old and have been updated with the updated Excel versions, but the formulas have always worked before.  Even a simple @sum is not working correctly...  This is time consuming and frustrating if I forget to recalculate.