Display Ajax ModalPopupExtender after PostBack in ASP.Net

whwn i click the button in modal popup my modal pop up dialog box is automatically close.actualluy i generate otp in my project. so when i clicked the generate otp bnutton this modal popup is closing. i dont to close my pop up right now. it should be remain until my all process should complete

<script src="assets/js/checkotp.js"></script>
    <script type="text/javascript">
    $(document).on("click",".menu",function(){

        $("#myModal").modal("show")
    })
    </script>

    <script type="text/javascript">
        $(document).on('click', '#btnsubmit', function (event)
    {
    event.preventDefault();
    var $modal = $('#myModal');
    $modal.find('#btnok').modal("show")
    });
    </script>

<div id="myModal" class="modal fade" data-backdrop="static" data-keyboard="false" role="dialog">
            <div class="modal-dialog">
                <div class="modal-content">
                    <div class="modal-header">
                        <button id="btnclose" type="button" class="close" data-dismiss="modal" aria-disabled="false" aria-hidden="true">&times;</button>                
                        <h1 class="modal-title panel-title" id="Modalsearch"><b><center>Contact</center></b></h1>
                    </div>
                    <div class="modal-body"><center>
                        <asp:TextBox ID="txtname" CssClass="form-control" Width="300" placeholder="Your Name" CausesValidation="true" runat="server"></asp:TextBox>
                        <asp:RequiredFieldValidator ID="RequiredFieldValidator1" ControlToValidate="txtname" ValidationGroup="NeededForValidate" runat="server" ErrorMessage="*"></asp:RequiredFieldValidator><br />
                        <asp:TextBox ID="txtemail" CssClass="form-control" Width="300" placeholder="Your Email" runat="server"></asp:TextBox>
                        <asp:RegularExpressionValidator ID="RegularExpressionValidator5" runat="server" ErrorMessage="*" ValidationGroup="NeededForValidate"  ControlToValidate="txtemail" ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*"></asp:RegularExpressionValidator><br />
                        <asp:DropDownList ID="ddlcontent" CssClass="form-control" runat="server" Width="300">
                            <asp:ListItem Value="post">Select Plan</asp:ListItem>     
                            <asp:ListItem Value="post">PostPaid</asp:ListItem> 
                            <asp:ListItem Value="brwl">Broadband with LandLine</asp:ListItem>
                            <asp:ListItem Value="brwol">Broadband without LandLine</asp:ListItem>
                            <asp:ListItem Value="ILL">ILL</asp:ListItem>
                            <asp:ListItem Value="tollfree">Tollfree Nos</asp:ListItem>
                            <asp:ListItem Value="PRI">PRI</asp:ListItem>
                        </asp:DropDownList><br />
                        <asp:TextBox ID="txtcity" CssClass="form-control" Width="300" placeholder="City" runat="server"></asp:TextBox><br />
                        <asp:TextBox ID="txtmob" CssClass="form-control" Width="300" MaxLength="10" onkeypress="return isNumberKey(event)" placeholder="Your Contact Nunmber" CausesValidation="true" runat="server"></asp:TextBox>
                        <asp:RequiredFieldValidator ID="RequiredFieldValidator2" ControlToValidate="txtmob" runat="server" ValidationGroup="NeededForValidate" ErrorMessage="*"></asp:RequiredFieldValidator><br /><br />
                        <asp:Button ID="btnsubmit" class="btn btn-warning" OnClick="btnsubmit_Click" ValidationGroup="NeededForValidate" runat="server" Text="Generate OTP" /><br />
                        <asp:TextBox ID="txtOTP" class="btn" MaxLength="4" Width="100" onkeyup="return Check()" Visible="false" placeholder="Enter OTP" runat="server"></asp:TextBox>
                        <asp:Button ID="btnok" class="btn btn-success" Visible="false" OnClick="btnok_Click" runat="server" Text="Confirm" />
                        <asp:Label ID="lblMessage" runat="server" Text=""></asp:Label>       
                    </div>                                                
                </div>
            </div>
        </div>
    <ajax:ModalPopupExtender ID="mpesubmit" runat="server" TargetControlID="btnDummy" PopupControlID="myModal" BackgroundCssClass="modalBackground"></ajax:ModalPopupExtender>

try
        {
            common load = new common();
            //For generating OTP
            Random r = new Random();
            string OTP = r.Next(1000, 9999).ToString();
            string Number = txtmob.Text;
            string Message = "Your One Time Password is " + OTP + ". Do not share this with anyone for security reasons.";
            string URL = "http://pay4sms.in/sendsms/?token=d9279885a4ceee392df8cf4208d0fc78&credit=3&sender=RADTEC&message=" + Message + "&number=" + Number + "";
            HttpWebRequest req = (HttpWebRequest)WebRequest.Create(URL);
            HttpWebResponse resp = (HttpWebResponse)req.GetResponse();
            StreamReader sr = new StreamReader(resp.GetResponseStream());
            string results = sr.ReadToEnd();
            sr.Close();
            txtname.Visible = false;
            txtemail.Visible = false;
            txtmob.Visible = false;
            txtcity.Visible = false;
            ddlcontent.Visible = false;
            txtOTP.Visible = true;
            btnok.Visible = true;
            
            string to = txtemail.Text; //To address    
            string from = "samasp27@gmail.com"; //From address    
            MailMessage message = new MailMessage(from, to);
            string mailbody = "You like to book New Connection " + ddlcontent.SelectedItem.ToString();
            message.Subject = "Get Your New Connection";
            message.Body = mailbody;
            //message.BodyEncoding = Encoding.UTF8;
            message.IsBodyHtml = true;
            SmtpClient client = new SmtpClient("smtp.gmail.com", 587); //Gmail smtp    
            System.Net.NetworkCredential basicCredential1 = new
            System.Net.NetworkCredential("samasp27@gmail.com", "9884289554");
            //
            client.EnableSsl = true;
            client.UseDefaultCredentials = false;
            client.Credentials = basicCredential1;
            client.Send(message);
            //Store the OTP in session to verify in next page.
            //If you want to verify from DB store the OTP in DB for verification. But it will take space
            Session["OTP"] = OTP;


            mpesubmit.Show();
            ////Session["name"] = txtname.Text;
            ////Session["Number"] = Number;
            ////Session["email"] = txtemail.Text;
            ////Session["plan"] = ddlcontent.SelectedItem.ToString();
            ////Session["city"] = txtcity.Text;
            //////Session["pincode"] = txtpincode.Text;
            ////string EncIP = HttpUtility.UrlEncode(load.EncryptUrl(OTP));
            ////Response.Redirect(string.Format("otp.aspx?Id={0}", EncIP));
        }
        catch (Exception ex)
        {
            lblMessage.Text = ex.Message.ToString();
        }