Essai ça :
Private Sub Worksheet_Change(ByVal Target As Range)
Dim KeyCells As Range
Dim wb As Workbook
Dim ws As Worksheet
Set KeyCells = Range("TA CELLULE")
If Not Application.Intersect(KeyCells, Range(Target.Address)) _
Is Nothing Then
Set wb = Workbooks.Open("C:\ICI LE CHEMIN DU FICHIER")
Set ws = wb.Worksheets(1)
End If
End Sub