ModalPopup extender not displaying PopUp

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C#

ASP.NET

I use the following code to display 'Panel1' as PopUp. When I click targetId button' BtnPopUp' nothing is happenning. The Panel1 is not getting displayed.

<asp:scriptmanager id="ScriptManager1" runat="server">
</asp:scriptmanager>
<asp:button id="Button1" runat="server" text="BtnPopUp" />
<asp:modalpopupextender id="ModalPopupExtender1" runat="server"
    cancelcontrolid="btnCancel" okcontrolid="btnOkay"
    targetcontrolid="Button1" popupcontrolid="Panel1"
    popupdraghandlecontrolid="PopupHeader" drag="true"
    backgroundcssclass="ModalPopupBG">
</asp:modalpopupextender>
<asp:panel id="Panel1" style="display: none" runat="server">
    <div class="HellowWorldPopup">
    <div class="PopupHeader" id="PopupHeader">Header</div>
    <div class="PopupBody">
    <p>This is a simple modal dialog</p>
    </div>
    <div class="Controls">
    <input id="btnOkay" type="button" value="Done" />
   <input id="btnCancel" type="button" value="Cancel" />
        </div>
        </div>
</asp:panel>