ACCENTA 8 - Door entry problem?

Hi,
Our Accenta8 system has just started doing an odd thing. All aspects of the system work fine accept one fault which has just occured this week. The exit door zone does not appear to be recgnised anymore. This is random as the system has worked without fault for years. Basically the alarm will set and unset fine, but the opening and closing of zone 1 (the exit door) does not trigger the the alarm etc. Can anyone help me identify the problem ?