Can anyone shed light why one of event procedures is not running.

I have a couple of workbooks whose formulae can link in either direction
depending on a control value held in a third "control" file. In order that
the status of that third file is always recognised I created a Workbook.Open
Event Procedure that Opens the control file if it is not already open. That
runs fine.(The procedure is held in Thisworkbook)

Recently I was fed up with getting too many divide by zero errors so I
included a small user-defined function in a code module to handle this. Now
my Workbook.Open Event procedure won't fire. If I remove the user-defined
function it fires. Confused? I was!

Anyone any ideas as to what is happening?