Are you deleting the cells of the named range somewhere in your code? The
named range will refer to 'Worksheet Name'!#REF! after the delete, and will
be invalid the next time you try to use it.

You will have to step through all of your code to figure out where this is
happening.

You might consider including an error hander and doing it like this:

Public Sub Test()
Dim rngNamedRange As Range

On Error Resume Next

Set rngNamedRange = ThisWorkbook.Names("NamedRange").RefersToRange

If rngNamedRange Is Nothing _
Then
MsgBox "'NamedRange' has disappeared!", vbCritical + vbOKOnly
Else
'Continue processing.
End If
End Sub

--
Regards,
Bill Renaud