Hi
Just a though, someone may have opened a file that has Auto Calculation
set to manual when opened and does not have it set back to Auto when
closing.
To check this, depending on the version you are using.
2000 - 2007.
Tools | Options | Calculation
Select Automatic.
2010
File | Options | Formulas | Calculation Options
Select Automatic.
Or if you want to keep the sheet as manual, just hit F9 to do an instant
auto-calc.
HTH
Mick.