Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
    MsgBox "123", vbInformation
End Sub

Option Explicit

Dim strCurrentValue$, blnAutoChange As Boolean

Private Sub Workbook_Open()
    strCurrentValue = Sheets("Sheet1").Cells( 1 ,  1 ).Value
End Sub

Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
    Dim lngResp&
    If Not blnAutoChange Then
        If Sh.Name = "Sheet1" Then
            If Target.Row =  1  And Target.Column =  1  Then
                If Target.Value <> "0" Then
                    lngResp = MsgBox("Not equal 0! Save or not?", vbYesNo + vbDefaultButton2)
                    If lngResp = vbNo Then
                        blnAutoChange = True
                        Target.Value = strCurrentValue
                    Else
                        strCurrentValue = Target.Value
                    End If
                End If
            End If
        End If
    Else
        blnAutoChange = False
    End If
End Sub