Excel formulas not updating

HI folks, I am not an excel professional so bear with me. 

I use a simple spreadsheet that we've developed over the years to enter certain data into and answer yes or no questions. Based on this information, formulas in other cells calculate costs and time. The end result is our estimate (Ballpark). This has served us well for many years... gradual improvements over that time.

Today the worksheet has decided not to automatically calculate the formulas. Old results remain after changing other cells. UNLESS I click on the cell with the formula (double click) and hit Enter. Then the cell updates the figures. Needless to say this is a pain in the butt. 

Anyone experience this before and is there a quick fix?

thanks in advance.

Jeff