ModalPopupExtender is not displaying

Below is my code and it is not working for ModalPopupExtender 

ModalPopupExtender window is not coming 

I dont know what is wrong into this ? 

can you please help me ? 

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" Runat="Server">
<Ajax:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
      </Ajax:ToolkitScriptManager>

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
      <ContentTemplate>
      <asp:GridView	id="Gridview1"
	        runat="server"   DataKeyNames="StudentID" 
	        PageSize="30" EmptyDataText = "No Records Found" EmptyDataRowStyle-HorizontalAlign ="Center"
            AllowPaging="true"  DataSourceID = "SqlDataSource1"
            HorizontalAlign="Center" BorderColor ="Black"   
            AllowSorting = "true"  Visible= "true" AutoGenerateColumns= "false"    >
             <HeaderStyle BackColor="#DCDCDC" Font-Bold="true" ForeColor="Black"  Font-Size = "Medium"  Font-Names="Calibri"   Wrap ="true"   Height = "30"   BorderColor ="Black" BorderStyle ="Groove" BorderWidth ="1.2" />
	        <RowStyle BackColor="AliceBlue"  Font-Size = "Small"  Font-Names="Calibri"  />
	        <AlternatingRowStyle BackColor="White"  Font-Size = "Small"  Font-Names="Calibri"/>
            <EmptyDataRowStyle  Font-Size = "Small" ForeColor ="Black" Font-Bold ="true"  HorizontalAlign = "Center" BackColor = "AliceBlue"  />
            <PagerStyle   BackColor="#DCDCDC"    ForeColor="Black" Font-Size="Large"   /> 
            <PagerSettings  Position="Bottom"  mode = "NumericFirstLast"  LastPageText = "Last Page" />

            <Columns>       

              <asp:BoundField DataField="School no" HeaderText="School" SortExpression =  "School No"  />
                <asp:TemplateField  HeaderText "StudentID">
                   <ItemTemplate  > 
                    <asp:LinkButton runat ="server" ID="LinkStudentID" Text='<%#Eval("StudentID") %>'  OnClick ="LinkStudentID_Click" SortExpression="StudentID"> </asp:LinkButton>
                                     </ItemTemplate>                    
                </asp:TemplateField>
                              <asp:BoundField DataField="Student Name" HeaderText="Student"  SortExpression =  "Student Name"/>
                             
              </Columns>

               
        </asp:GridView>


            </ContentTemplate>
      </asp:UpdatePanel>


       <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
           ConnectionString="<%$ appSettings:DCconnString  %>"         
           SelectCommandType ="StoredProcedure"  SelectCommand = "StudentPRoc"   >
       
       </asp:SqlDataSource>

 

       <br />
    
     <asp:Button runat="server" ID="btnModalPopUp"  style="display:none" 
           />

      <Ajax:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID= "btnModalPopUp" PopupControlID ="ModalpopupPanel" OkControlID ="btnOK" X ="20" Y ="20" >
      </Ajax:ModalPopupExtender>

      <asp:Panel ID="ModalpopupPanel" runat="server" BackColor="Red">
    Testing for modal pop up window 
      <asp:Button ID="ModalpopupButton" runat="server"  Text="Close" />
    </asp:Panel>

</asp:Content>
   Protected Sub LinkStudentID_Click(ByVal sender As Object, ByVal e As EventArgs) 'Handles LinkStudentID.Click
        Try
       
        ModalPopupExtender1.Show()
        Dim lnkStudentid As LinkButton = TryCast(sender, LinkButton)
            Dim strstudentid As String = lnkStudentid.Text
        Catch ex As Exception
        
        End Try
    End Sub