Model Pop up disappears

<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:Button ID="btnShow" runat="server" Text="Show Modal Popup" />

<!-- ModalPopupExtender -->
<cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panel1" OkControlID="x" TargetControlID="btnShow"
    CancelControlID="btnClose" BackgroundCssClass="modalBackground">
</cc1:ModalPopupExtender>
<asp:Panel  ID="Panel1" runat="server"  align="center" style = "display:none">
    This is an ASP.Net AJAX ModalPopupExtender Example<br />
    <asp:Button ID="btnClose" runat="server" Text="Close" />
</asp:Panel>
</asp:Content>