Excel isn't calculating the SUM of cells [duplicate]

I am trying to use a very simple formula which is =SUM(B9:B11). However the cell doesn't compute for some reason.

I've used Excel for years and have never had this problem. Any idea why it may be failing to update the SUM?

I'm using Excel 2007 on Windows 7 Pro. I've opened the spreadsheet on multiple different machines with the same results so it appears to be an issue with the spreadsheet itself and not Excel or the computer.

Additional Note: If I recreate the =SUM formula it will recompute the total. However, if I change one of the number it still doesn't auto-recalculate.

Also, if I press F9 the SUM will recalculate being manually forced to.

Also do you have formula calculation set to Manual?

This happened to me when I changed my computer's default language from English to French. In French, commas (,) are used instead of decimal points (.) in numbers, and vice versa. In Excel I was still writing numbers in the English format (3.42) when it was expecting a French format (3,42).

I fixed it by finding 'Change the date, time or number format' in the Start search box, clicking on Additional Settings, then changing the symbol for a decimal point to (.) instead of (,).

If a cell contains a circular reference, it will not autosum correctly (or at all). From the Formula tab, choose Error Checking>Circular Reference, trace it and then fix it.