Display a modal popup using AJAX

Question

I have designed a modal popup in ASP.net. The task is when I click my button, it has to display a modal popup box. I used AJAX extender control kit also. Below is my coding.

Approval.aspx:

<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server"/>
<div>
    <asp:Button ID="ApproveButton" runat="server" Text="Approve" />

    <ajaxToolkit:ModalPopupExtender ID ="mp1" runat="server" PopupControlID="Panel1"
                 TargetControlID="ApproveButton" CancelControlID="CancelButton">
                 </ajaxToolkit:ModalPopupExtender> 
    <!--Modal Panel "Panel1"-->
    <asp:Panel ID="Panel1" runat="server" Height="170px" Width="506px">
        &nbsp;
        <table style="width: 100%; height: 164px;">
            <tr>
                <td>
                    <asp:Label ID="ReasonLabel" runat="server" Text="Reason for Approval"></asp:Label>
                </td>
                <td>
                    <asp:TextBox ID="ReasonTextBox" runat="server" Height="126px" 
                        TextMode="MultiLine" Width="336px"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                    &nbsp;</td>
                <td>
                    <asp:Button ID="OkButton" runat="server" Text="Ok" Width="66px" />
                    &nbsp;&nbsp;&nbsp;&nbsp;
                    <asp:Button ID="CancelButton" runat="server" Text="Cancel" />
                </td>
            </tr>
        </table>
    </asp:Panel>
    <!--Modal Panel-->
</div>

The problem is, the Popup window is just blinking once when I clicked the button. It is not displaying the popup permanently.