Cannot redirect after HTTP headers have been sent.

I have created one page to show the progress of payment . After successful payment i need to redirect user to the payment confirmation page. When i am redirecting am getting the following error.
Cannot redirect after HTTP headers have been sent.
Can anyone help me to solve this problem?
Please find my code below.
Imports System.IO Partial Public Class ProcessingPayment Inherits System.Web.UI.Page Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load System.Threading.Thread.Sleep(10000) Response.Redirect("paymentconfirmation.aspx") End Sub Protected Overrides Sub OnLoad(ByVal e As EventArgs) If Not IsPostBack Then Dim fileName As String = System.Web.Configuration.WebConfigurationManager.AppSettings("Progresspath").ToString() fileName = Server.MapPath(fileName) If File.Exi ...