Hi Balakrishna
It was not working. B'cos you didn't close the modalpopupextender tag
I have just closed the tag and executed, It is workin.
Below is the modified code:
<cc1:modalpopupextender id="ModalPopupExtender1" backgroundcssclass="ModalPopupBG"
runat="server" cancelcontrolid="btnCancel" okcontrolid="btnOkay" targetcontrolid="Button1"
popupcontrolid="Panel1" drag="true" popupdraghandlecontrolid="PopupHeader"></cc1:modalpopupextender>
<div id="Panel1" style="display: none;" class="popupConfirmation">
<div class="popup_Container">
<div class="popup_Titlebar" id="PopupHeader">
<div class="TitlebarLeft">Popup Header</div>
<div class="TitlebarRight"></div>
</div>
<div class="popup_Body">
<p>
This is a simple modal dialog
</p>
</div>
<div class="popup_Buttons">
<input id="btnOkay" value="Done" type="button" />
<input id="btnCancel" value="Cancel" type="button" />
</div>
</div>
</div>