Modal Popup Ajax Extender is not working

<%@ Page Language="C#" MasterPageFile="~/Main.Master" AutoEventWireup="true" CodeBehind="Menu.aspx.cs" Inherits="UserPanel.Menu" Title="Untitled Page" %>

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>

<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<style type="text/css">
    .modalBackground
    {
        background-color: Black;
        filter: alpha(opacity=90);
        opacity: 0.8;
    }
    .modalPopup
    {
        background-color: #FFFFFF;
        border-width: 3px;
        border-style: solid;
        border-color: black;
        padding-top: 10px;
        padding-left: 10px;
        width: 300px;
        height: 140px;
    }
</style>


  <asp:ScriptManager ID="ScriptManager1" runat="server"/>	          
                <div class="row">
                    <div class="col-md-12">
                        <h1 class="page-head-line">Menu</h1>
                        <h1 class="page-subhead-line">Select Your Menu !</h1>

                    </div>
                </div>
                
                <!-- /. ROW  -->
                <div class="row">
            
               <div class="panel panel-info">
                        <div class="panel-heading">
                           *You Can Place On Order at a Time !
                        </div>
                      
                        <div align="center">
<asp:LinkButton ID="LB_Sheesha"  runat="server">Order Normal Shesha <br />
 <asp:LinkButton ID="LB_Special" runat="server">Order Special Shesha
                        </div>
                      
      </div>
    <asp:Panel ID="Panel1" runat="server" visible=false  >
               
      <table>
      <tr>
      <td>
          <asp:Label ID="Label1" runat="server" Text="Label"> Select Flavour
      
      </td>      
      <td>
      
          <asp:DropDownList ID="Drop_nsheesha" runat="server">
          
          
      </td>      
     </tr></table>
     
    <asp:Button ID="B_Next" class="btn btn-info" runat="server" Text="Next" />
   <asp:Button ID="B_Cancel" class="btn btn-info" runat="server" Text="Cancel" /> 
                            
     
     </div>
     
    <asp:ModalPopupExtender ID="ModalPopupExtender2" runat="server"
      TargetControlID="LB_Sheesha" PopupControlID="Panel1" DropShadow="true" 
        OkControlID="B_Next"  CancelControlID = "B_Cancel" 
        OnOkScript="onOK()" BackgroundCssClass="modalBackground" 
        oncancelscript="Are You Sure ? " 
                      >