Excel Mac Multiply Function Stopped Working

I have been using Excel Mac 2011 for 4 years and never had this problem before but suddenly i can't get the multiply function to work. I have opened up numerous spreadsheets that have the formula =A1*A2 but they seem to not be working anymore either.

Does anyone have any ideas what has happened?

No idea, but you are pretty sparse with the details.

I assume these are old files and you have not changed the formula. You used to see a result, but now you don't. Is that correct?

There are other formulas not using multiplication that work. Right?

You've tried to do a new multiplication, but it failed also. Correct?

All of the above, is, of course, very unusual.

Did you recently upgrade your OS? Add any other programs?