Restricting the Page from being Postback

<style type="text/css">
    .HideBackGround
    {
    background-color: #696969;
    filter: alpha(opacity=40);
    opacity: 0.7;
    z-index:-1;
    }
</style> 

<asp:ScriptManager ID="script1" runat="server"></asp:ScriptManager>     
<asp:UpdateProgress ID="UpdateProgress" runat="server">
<ProgressTemplate>
<asp:Image ID="img_Progress1" ImageUrl="images/loading.gif" AlternateText="Processing" runat="server" />
</ProgressTemplate>
</asp:UpdateProgress>
<cc1:ModalPopupExtender ID="modalPopup" runat="server" TargetControlID="UpdateProgress"
PopupControlID="UpdateProgress" BackgroundCssClass="HideBackGround" />
<asp:UpdatePanel ID="pnlData" runat="server" UpdateMode="Conditional" >
<ContentTemplate>

Your Content

 </ContentTemplate>
    </asp:UpdatePanel>
    <script type="text/javascript">
        var prm = Sys.WebForms.PageRequestManager.getInstance();
        //Raised before processing of an asynchronous postback starts and the postback request is sent to the server.
        prm.add_beginRequest(BeginRequestHandler);
        // Raised after an asynchronous postback is finished and control has been returned to the browser.
        prm.add_endRequest(EndRequestHandler);
        //     function load() {
        //         Sys.WebForms.PageRequestManager.getInstance().add_endRequest(BeginRequestHandler);
        //         Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
        //     }
        function BeginRequestHandler(sender, args) {
            //Shows the modal popup - the update progress
            var popup = $find('<%= modalPopup.ClientID %>');
            if (popup != null) {
                popup.show();
            }
        }

        function EndRequestHandler(sender, args) {
            //Hide the modal popup - the update progress
            var popup = $find('<%= modalPopup.ClientID %>');
            if (popup != null) {
                popup.hide();
            }
        }
</script> 

Show Alert Message in a page with UpdatePanel

 Inside App_Code     


       public static void ShowMessage(string message)
        {
            Page currentPage = HttpContext.Current.CurrentHandler as Page;

            //If the calling page has updatePanel  
       
            ScriptManager.RegisterClientScriptBlock(currentPage, currentPage.GetType(), "showalert", "alert('" + message + "');", true);
           
           //If the calling page does not have updatePanel

           // ScriptManager.RegisterStartupScript(currentPage,currentPage.GetType(), "showalert", "alert('" + message + "');", true);
        }




If Inside the same Page


      public void showMessage(string Message)
        {
            ScriptManager.RegisterStartupScript(this, GetType(), "showalert", "alert('"+Message+"');", true);

        }