ModalPopupExtender1.Show(); not working in server side

<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;
        txtCity.Text = gvrow.Cells[4].Text;
        ModalPopupExtender1.Show();
    }