بفرما مشکلش حل شد :
Option Strict Off
Option Explicit On
Public Class Form1
Inherits System.Windows.Forms.Form
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hWnd As Integer, ByVal wMsg As Integer, ByVal wParam As Integer, ByVal lParam As Int32) As Integer
Const SC_MONITORPOWER As Integer = &HF170
Const MON_OFF As Short = 2
Const WM_SYSCOMMAND As Short = &H112S
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
SendMessage(Me.Handle.ToInt32, WM_SYSCOMMAND, SC_MONITORPOWER, MON_OFF)
End Sub
End Class