Hi Viji,
Please make sure that your ModalPopupExtender's PopupControlID,TargetControlID,CancelControlID,OKControlID (the controls you tied to these properties) are should be in the same container. (CancelControlID and OKControlID are not necessary ) Otherwise, ModalPopupExtender won't find it. For example, if Button1 is the TargetControlID and it is inside a TabContainer. It won't work in this situation.
Best regards,
Jonathan
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