Your dlookup isn't right.
DLookup («expr», «domain», «criteria»)
«expr» : the table field you want to return
«domain» : table
«criteria» : where clause - somefield = somevalue
=DLookUp("[FORWRD_NAME]","File_No","[FILE_NO] = Form![FileNo]")
should be (assuming File_No is text, if not remove the single quotes)
=DLookUp("[FORWRD_NAME]","File_No","[FILE_NO] = '" & [Forms]![File_No]![File_No] & "'")
also assuming you have a table called File_No containing a field called File_No and you're getting a value from a control called File_No on a form called File_No.