Regarding Modal Popup Extender

<asp:Button ID="btnShowPopup" runat="server" Style="display: none" />
    <asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" 
        TargetControlID="btnShowPopup" BehaviorID="ModalBehaviour" 
        PopupControlID="pnlpopup" CancelControlID="btnCancel"
        BackgroundCssClass="modalBackground">
    </asp:ModalPopupExtender>
    <asp:Panel ID="pnlpopup" runat="server" BackColor="White" 
        Height="269px" Width="400px" Style="display: none">

protected void imgbtn_Click(object sender, ImageClickEventArgs e)
{
    ImageButton btndetails = sender as ImageButton;
    GridViewRow gvrow = (GridViewRow)btndetails.NamingContainer;
    lblID.Text = gvdetails.DataKeys[gvrow.RowIndex].Value.ToString();
    lblusername.Text = gvrow.Cells[1].Text;
    txtfname.Text = gvrow.Cells[2].Text;
    txtlname.Text = gvrow.Cells[3].Text;
    txtEmail.Text = gvrow.Cells[4].Text;
    ModalPopupExtender1.Show();
    //pnlpopup.Style.Add("display", "block");
}