ModalPopupExtender disappears on specifyin TargetControlID

Hi All,

I have a ModalPopUPextender in my page. ALso the Popup is supposed to be a Panel which has textboxes, Labels, Dropdowns. I also have a link button which I want to use to bring up the pop up. The problem is during design time when I mention the TargetControlID of the Link button, the ModalPopUpExtender disappears from the page. Also while runtime, I can't see the Link button also.

Thanks in advance.