after update stopped working

Hello I wonder if any one can help me.
ms access 2000, pro win xp home
I use a form in datasheet view to enter dates of death, ages of death and
date of birth.
I calculate year of birth if no date of birth by the following code in the
after update event of the year of birth field
YoB = YEAR(DoD) - AoD
Year of birth = year of death - age of death -- in english
this worked fine until a few days ago and has now stopped working.
Any help will be greatly appreciated
Thanks in advance