Modalpopupextender.show() not firing

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ASP.NET

HI,
I am working on asp:ModalPopupExtender.
on clicking an image button a pop-up should open.
But pop-up is not opening.

What I have tried:

<asp:UpdatePanel ID="UpdatePanel3" runat="server">
                                                                <ContentTemplate>
                                                                   <%-- <asp:DropDownList ID="ddlemdtype" runat="server" Style="margin-left: 29px; width: 188px;"
                                                                        class="inputddl" AutoPostBack="true" OnSelectedIndexChanged="ddlemdtype_SelectedIndexChanged"
                                                                        TabIndex="17">
                                                                    </asp:DropDownList>--%>
                                                                    <span class="mandatoryfield" style="margin: 5px 0 0 0; position: absolute;">*</span>
                                                                    <%--<asp:ImageButton ID="ImageButtonEMDType" ImageUrl="~/images/add.png" runat="server"
                                                                        Style="margin: 7px 0 0 8px; position: absolute;" AlternateText="Add New" OnClick="imgbtnaddnew_Click" />--%>
                                                                    <div>
                                                                        <asp:LinkButton ID="LinkButton3" ImageUrl="~/images/cross.png" runat="server" Style="display: none">
                                                                        </asp:LinkButton>
                                                                        <asp:ModalPopupExtender ID="mdlsample" runat="server" TargetControlID="LinkButton3"
                                                                            PopupControlID="_pnlEmdType">
                                                                        </asp:ModalPopupExtender>
                                                                        <asp:Panel ID="_pnlEmdType" runat="server" ScrollBars="Auto" Style="margin-right: -10px;
                                                                            display: none;">
                                                                            <div id="DivLeadType" style="float: right;">
                                                                                <asp:ImageButton ID="ImageButton2" runat="server" ImageAlign="Right" ImageUrl="~/images/popup_close_button.png"
                                                                                    Width="28px" Height="28px" />
                                                                                <br />
                                                                                <div style="background-color: #CCCCCC; margin-right: 12px; border: solid 6px #ffffff;">
                                                                                    <div style="background-color: #CCCCCC; border: solid 4px #ffffff;">
                                                                                        <table border="0" width="350" align="center" cellpadding="0" cellspacing="0">
                                                                                            <tr>
                                                                                                <td class="popuptitle" align="center">
                                                                                                    
                                                                                                        <asp:Label ID="lblpopmastertitle" runat="server" Text="EMD Type"></asp:Label>
                                                                                                </td>
                                                                                            </tr>
                                                                                            <tr>
                                                                                                <td align="center" class="popupcontent">
                                                                                                    <table>
                                                                                                        <tr>
                                                                                                            <td>
                                                                                                                <asp:Label ID="lblpopupRowname" runat="server" Text="EMD Type"></asp:Label>
                                                                                                            </td>
                                                                                                            <td>
                                                                                                                <asp:TextBox ID="txtpopEmdType" runat="server" class="input" MaxLength="500"></asp:TextBox>
                                                                                                                <span class="mandatoryfield">*</span>
                                                                                                            </td>
                                                                                                        </tr>
                                                                                                    </table>
                                                                                                </td>
                                                                                            </tr>
                                                                                            <tr>
                                                                                                <td class="popupfooter" colspan="2">
                                                                                                    <asp:Button ID="btnpopupSave" runat="server" Text="Save" OnClick="btnpopupSave_OnClick"
                                                                                                        TabIndex="35" OnClientClick="return CheckValidation();" />
                                                                                                    <asp:Button ID="btnpopupAppClose" runat="server" Text="Cancel" TabIndex="35" /> 
                                                                                                </td>
                                                                                            </tr>
                                                                                        </table>
                                                                                    </div>
                                                                                </div>
                                                                            </div>
                                                                        </asp:Panel>
                                                                    </div>
                                                                </ContentTemplate>
                                                               
                                                                <Triggers>
                                                                    <asp:PostBackTrigger ControlID="ddlLeadStatus" />
                                                                </Triggers>
                                                            </asp:UpdatePanel>