It is a good practice to find the specific window before you use the AppActivate function.
Code:
Option Explicit
Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As Long, ByVal lpWindowName As String) As Long
Private Declare Function ShowWindow Lib "user32" (ByVal hwnd As Long, ByVal nCmdShow As Long) As Long
Private Const SW_SHOWNORMAL = 1
Dim xhwnd&
xhwnd = FindWindow(0&, <Your Application Window Title>)
If xhwnd <> 0 Then
AppActivate <Your Application Window Title>
xhwnd = ShowWindow(xhwnd, SW_SHOWNORMAL)
End
End If
End If