The button click event fires Modalpopupextender but does not turn off

Question

MODAL appears, but NO longer wants to leave! How do I fix this?

I have a code in ASP.Net, . Net Framework 4 with Visual Studio 2010.

<asp:button ID="hButton1" runat="server" Style="display:none;"/>

<asp:Button ID="btnUploadArq" runat="server" CssClass="Botao" Text="Carregar Arquivo"
    Style="height: 5em; position: absolute; z-index: 99999!important;" ToolTip="Enviar o arquivo selecionado"
    TabIndex="2" OnClick="_btnUploadArq_Click" />

<ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="hButton1"
    PopupControlID="updateProgress" BackgroundCssClass="modalBackground">
</ajaxToolkit:ModalPopupExtender>

I need to place a MODAL form waiting message (Modalpopupextender1) for the user:

protected void _btnUploadArq_Click(object sender, EventArgs e)
{
   ModalPopupExtender1.Show();

   if (ArqUpload.HasFile)
       ImportaArq(ArqUpload.PostedFile);
}

I followed the example of the page:

Modalpopupextender OK Button click Event not Firing:

Modalpopupextender calling . Show () does not work