Use the ShowWindow API.
Code:
Private Declare Function ShowWindow Lib "user32" (ByVal hwnd As Long, ByVal nCmdShow As Long) As Long
Private Declare Function FindWindowEx Lib "user32" Alias "FindWindowExA" (ByVal hWnd1 As Long, ByVal hWnd2 As Long, ByVal lpsz1 As String, ByVal lpsz2 As String) As Long

Private Sub Command1_Click()
    Dim hApp As Long
    hApp = FindWindowEx(0, 0, "SciCalc", "Calculator")
    If hApp <> 0 Then ShowWindow hApp, 0 '<-- Change 0 to 1 to display the Window again
End Sub