Hi arvindasp,

If you dont want to refresh the page on Continue button click then you need to use AJAX to call WebMethod from server side instead of using window.location.

I am using this article and modified the code as per your requirement.

Display Session Timeout message before Session expires in ASP.Net

First, you need to set the EnablePageMethods to TRUE inside the ScriptManager.

Please refer the below code.

HTML 

<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">
</asp:ScriptManager>
<h3>Session Idle:&nbsp;<span id="secondsIdle"></span>&nbsp;seconds.</h3>
<asp:LinkButton ID="lnkFake" runat="server" />
<asp:ModalPopupExtender ID="mpeTimeout" BehaviorID="mpeTimeout" runat="server" PopupControlID="pnlPopup"
    TargetControlID="lnkFake" OkControlID="btnYes" CancelControlID="btnNo" BackgroundCssClass="modalBackground"
    OnOkScript="ResetSession()">
</asp:ModalPopupExtender>
<asp:Panel ID="pnlPopup" runat="server" CssClass="modalPopup" Style="display: none">
    <div class="header">
        Session Expiring!
    </div>
    <div class="body">
        Your Session will expire in&nbsp;<span id="seconds"></span>&nbsp;seconds.<br />
        Do you want to reset?
    </div>
    <div class="footer" align="right">
        <asp:Button ID="btnYes" runat="server" Text="Yes" CssClass="yes" />
        <asp:Button ID="btnNo" runat="server" Text="No" CssClass="no" />
    </div>
</asp:Panel>

jQuery AJAX

<script type="text/javascript">
    var interval = null;
    var timeOutPopup = null;
    var timeOutRedirect = null;
    function SessionExpireAlert(timeout) {
        var seconds = timeout / 1000;
        document.getElementsByName("secondsIdle").innerHTML = seconds;
        document.getElementsByName("seconds").innerHTML = seconds;
        interval = setInterval(function () {
            seconds--;
            document.getElementById("seconds").innerHTML = seconds;
            document.getElementById("secondsIdle").innerHTML = seconds;
        }, 1000);
        timeOutPopup = setTimeout(function () {
            //Show Popup before 20 seconds of timeout.
            $find("mpeTimeout").show();
        }, timeout - 20 * 1000);
        timeOutRedirect = setTimeout(function () {
            window.location = "Expired.aspx";
        }, timeout);
    };
    function ResetSession() {
        //Redirect to refresh Session.
        //window.location = window.location.href;
        PageMethods.ResetSession(OnSuccess);
        function OnSuccess(response, userContext, methodName) {
            if (interval != null) {
                clearInterval(interval);
            }
            if (timeOutPopup != null) {
                clearTimeout(timeOutPopup);
            }
            if (timeOutRedirect != null) {
                clearTimeout(timeOutRedirect);
            }
            SessionExpireAlert(response);
        }
    };
</script>

Namespace

C#

using System.Web.Services;
using System.Configuration;
using System.Web.Configuration;

VB.Net

Imports System.Web.Services
Imports System.Configuration
Imports System.Web.Configuration

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    Response.Cache.SetCacheability(HttpCacheability.NoCache);
    if (!this.IsPostBack)
    {
        Session["Reset"] = true;
        Configuration config = WebConfigurationManager.OpenWebConfiguration("~/Web.Config");
        SessionStateSection section = (SessionStateSection)config.GetSection("system.web/sessionState");
        int timeout = (int)section.Timeout.TotalMinutes * 1000 * 30;
        ClientScript.RegisterStartupScript(this.GetType(), "SessionAlert", "SessionExpireAlert(" + timeout + ");", true);
    }
}


[WebMethod]
public static int ResetSession()
{
    HttpContext.Current.Session["Reset"] = true;
    Configuration config = WebConfigurationManager.OpenWebConfiguration("~/Web.Config");
    SessionStateSection section = (SessionStateSection)config.GetSection("system.web/sessionState");
    int timeout = (int)section.Timeout.TotalMinutes * 1000 * 30;
    return timeout;
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    Response.Cache.SetCacheability(HttpCacheability.NoCache)
    If Not Me.IsPostBack Then
        Session("Reset") = True
        Dim config As Configuration = WebConfigurationManager.OpenWebConfiguration("~/Web.Config")
        Dim section As SessionStateSection = CType(config.GetSection("system.web/sessionState"), SessionStateSection)
        Dim timeout As Integer = CInt(section.Timeout.TotalMinutes) * 1000 * 30
        ClientScript.RegisterStartupScript(Me.[GetType](), "SessionAlert", "SessionExpireAlert(" & timeout & ");", True)
    End If
End Sub

<WebMethod>
Public Shared Function ResetSession() As Integer
    HttpContext.Current.Session("Reset") = True
    Dim config As Configuration = WebConfigurationManager.OpenWebConfiguration("~/Web.Config")
    Dim section As SessionStateSection = CType(config.GetSection("system.web/sessionState"), SessionStateSection)
    Dim timeout As Integer = CInt(section.Timeout.TotalMinutes) * 1000 * 30
    Return timeout
End Function

Screenshot

Check this example, I have used UpdatePanel

<h3>Session Idle:&nbsp;<span id="secondsIdle"></span>&nbsp;seconds.</h3>
<asp:LinkButton ID="lnkFake" runat="server" />
<asp:ModalPopupExtender ID="mpeTimeout" BehaviorID="mpeTimeout" runat="server" PopupControlID="pnlPopup"
    TargetControlID="lnkFake" CancelControlID="btnNo" BackgroundCssClass="modalBackground">
</asp:ModalPopupExtender>
<asp:Panel ID="pnlPopup" runat="server" CssClass="modalPopup" Style="display: none">
    <div class="header">
        Session Expiring!
    </div>
    <div class="body">
        Your Session will expire in&nbsp;<span id="seconds"></span>&nbsp;seconds.<br />
        Do you want to reset?
    </div>
    <div class="footer" align="right">
        <asp:UpdatePanel runat="server">
            <ContentTemplate>
                <asp:Button ID="btnYes" runat="server" Text="Yes" CssClass="yes" OnClick="ResetSession" />
                <asp:Button ID="btnNo" runat="server" Text="No" CssClass="no" />
            </ContentTemplate>
        </asp:UpdatePanel>
    </div>
</asp:Panel>
<script type="text/javascript">
    var interval = null;
    var timeOutPopup = null;
    var timeOutRedirect = null;
    function SessionExpireAlert(timeout) {
        if (interval != null) {
            clearInterval(interval);
        }
        if (timeOutPopup != null) {
            clearTimeout(timeOutPopup);
        }
        if (timeOutRedirect != null) {
            clearTimeout(timeOutRedirect);
        }
        $find("mpeTimeout").hide();
        var seconds = timeout / 1000;
        document.getElementsByName("secondsIdle").innerHTML = seconds;
        document.getElementsByName("seconds").innerHTML = seconds;
        interval = setInterval(function () {
            seconds--;
            document.getElementById("seconds").innerHTML = seconds;
            document.getElementById("secondsIdle").innerHTML = seconds;
        }, 1000);
        timeOutPopup = setTimeout(function () {
            //Show Popup before 20 seconds of timeout.
            $find("mpeTimeout").show();
        }, timeout - 20 * 1000);
        timeOutRedirect = setTimeout(function () {
            window.location = "Expired.aspx";
        }, timeout);
    };
</script>

protected void Page_Load(object sender, EventArgs e)
{
    Response.Cache.SetCacheability(HttpCacheability.NoCache);
    if (!this.IsPostBack)
    {
        Session["Reset"] = true;
        Configuration config = WebConfigurationManager.OpenWebConfiguration("~/Web.Config");
        SessionStateSection section = (SessionStateSection)config.GetSection("system.web/sessionState");
        int timeout = (int)section.Timeout.TotalMinutes * 1000 * 60;
        ClientScript.RegisterStartupScript(this.GetType(), "SessionAlert", "window.onload = function(){ SessionExpireAlert(" + timeout + "); };", true);
    }
}

protected void ResetSession(object sender, EventArgs e)
{
    Session["Reset"] = true;
    Configuration config = WebConfigurationManager.OpenWebConfiguration("~/Web.Config");
    SessionStateSection section = (SessionStateSection)config.GetSection("system.web/sessionState");
    int timeout = (int)section.Timeout.TotalMinutes * 1000 * 60;
    ScriptManager.RegisterStartupScript(this, this.GetType(), "SessionAlert", "SessionExpireAlert(" + timeout + ");", true);
}