What is the code you've written?

This is the code I wrote.

Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As
Boolean)
Dim row As Integer
Dim fname As String
Dim group As String
On Error GoTo error
If Cells(row, 2).Value = "Combo" Then
group = "CM"
Else
group = "RS"
End If

row = Target.row

fname = ThisWorkbook.path & group & "/" & Cells(row, 3).Value & "/" & _
Cells(4, row) & "__" & Cells(5, row) & "__" & Cells(6, row) & ".txt"
FileName = fname
Exit Sub
error: // used for trying to trap  the error (couldn't trap)
Exit Sub
End Sub

The culprit was found!
The expression: Cells(row, 2) was called with row=0.
For some obscure reason the error trapping started to work  only after I closed
excel and re-invoked it.