on error resume next
set x=workbooks("MyBook.xls")
on error goto  0 
if x is nothing then ...

dim x as workbook
dim strfull$
dim strshort$
strfull$="C:\Base.xls"
strshort$=dir(strfull$)
if strshort$="" then exit sub
on error resume next
set x=workbooks(strshort$)
on error goto  0 
if x is nothing then set x=workbooks.open(strfull$)

Sub Save_Base ()
Dim iFileName$
iFileName$ = "D:\Base.xls"

If Dir(iFileName$) <> "" Then
With Application
.ScreenUpdating = False
With .Workbooks.Open(Filename:=iFileName$)
.Worksheets( 1 ).Range("A1:F10").Value = Workbooks("Work_book.xls").Worksheets( 1 ).Range("A1:F10").Value

.Close saveChanges:=True
End With
.ScreenUpdating = True
End With
Else
MsgBox "Спасибо что вы воспользовались услугами Аэрофлота", , _
"Русские авиалинии !!!"
End If
End Sub