ModalPopupExtender1.Show();窗口无法弹出

       <asp:UpdatePanel ID="UpdatePanel1" runat="server"  >
            <ContentTemplate>
            
             <asp:ModalPopupExtender  TargetControlID="HiddenField1" BackgroundCssClass="bg" 　PopupControlID="Pan_tan" ID="ModalPopupExtender1" runat="server">
            </asp:ModalPopupExtender>
            <asp:HiddenField ID="HiddenField1" runat="server" />
            <asp:Panel ID="Pan_tan" runat="server" Width="100%" Style="display: none">
             <table style=" width:500px;" class="modalPopup-text">
                                <tr>
                                    <td colspan="4">
                                        <strong>用户订单操作</strong></td>
                                </tr>
                                <tr>
                                    <td align="right" style="width: 100px">
                                        接收情况：</td>
                                    <td align="left" style="width: 100px">
                                        <asp:RadioButton ID="Radio_Y" runat="server" GroupName="aa" 
                                            Text="同意接收" /></td>
                                    <td style="width: 100px" align="left">
                                        <asp:RadioButton ID="Radio_N" runat="server" GroupName="aa" 
                                            Text="不同意接收" /></td>
                                    <td style="width: 100px">
                                        </td>
                                </tr>
                            
                            </table>
                      
                            </asp:Panel>
后台代码：
 ModalPopupExtender1.Show();现在遇到的问题是遮罩层始终不弹出来，麻烦各位帮帮忙了！