Private Sub Workbook_Open()
Call IsBookOpen("Бла.xls", False)
Workbooks("ЗапускБла.xls").Close (False)
End Sub
Function IsBookOpen(wbFullName, Err As String) As Boolean
    Dim wbBook As Workbook
    For Each wbBook In Workbooks
        If wbBook.Name = "Бла.xls" Then
           MsgBox "Файл 'Бла'" & " уже открыт," & Chr(10) & " запустите ОСНОВНУЮ ФОРМУ"
                     Err = 1
           ActiveWorkbook.Close
           GoTo l
        End If
    Next wbBook
'ChDir "\\Main\users\Яна\отчеты"
      Workbooks.Open Filename:="\\Main\users\Яна\отчеты\Бла.xls"
   
l:
End Function

Workbooks.Open Filename:="\\Main\users\Яна\отчеты\Бла.xls"