Why does Excel not auto-update my equations anymore?

For some reason all of a sudden when I update cells in Excel, my equations are not being updated.  For example, I have a simple equation =SUM(C4:C16).  If I change any of the cells in C4-C16, the answer doesn’t get updated…  I don’t know what happened I’ve never seen this fail before.  Any ideas?

Solution:

Check to make sure Automatic Calculation is still selected.  Go to Formulas->Calculation->Calculation Options and select “Automatic”.