popup window not stay on the screen

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ASP.NET

Javascript

Hi all,

Javascript function for popup:

function show_popup() {
            $find("InstrucionsCancelbtn").show();(InstrucionsCancelbtn - is ajax popup behaviorid)
        }

Design page:
<br />
 Should problem persist plesse <u><a href="AboutUs.aspx" style="color: Red" onclick="show_popup()">
                                                        Contact Us</a></u> for more Information.<br />
                                                    <br />

Ajaxpopup:

<ajaxtoolkit:ModalPopupExtender ID="ModalPopupExtenderID"  runat="server" BehaviorID="InstrucionsCancelbtn"
    TargetControlID="InstructionsHyperLink" PopupControlID="InstructionsPanelId"
    BackgroundCssClass="modalBackground">

<asp:Panel ID="InstructionsPanelId" runat="server" CssClass="modalPopup">
    <div class="inner" id="PopUpInstructions">
    </div>
    <table>
        <tr valign="top" style="padding-removed 400px;">
            <td style="width: 100px">
                Description:
            </td>
            <td style="width: 230px">
                <asp:TextBox ID="txtDescription" Height="50px" Width="230px" CssClass="text text_Box-large"
                    Wrap="true" runat="server" word-wrap="word-break;" TextMode="MultiLine">
                <asp:Label ID="lblalert" runat="server" ForeColor="Red" Text="">
            </td>
        </tr>
        <tr valign="middle">
            <td align="center" colspan="2" valign="middle" style="width: 100%;">
                <img id="imgsavecomments" title="Save"  runat="server" src="~/Images/ico_save.gif"
                    alt="Save"  önclick="javascript:AddDescriptionPopup();" />
                <img id="imgdeletecomments"  runat="server" title="Delete" src="~/Images/ico_delete.gif"
                    alt="Delete"  önclick="javascript:DeleteDescriptionPopup();" />
            </td>
        </tr>
    </table>
    <asp:HyperLink ID="InstructionsHyperLink" runat="server" OnClick="Description();"
        Font-Underline="True" ForeColor="Blue" />

If I click the contact us, I need to open the popup window, this is the code and popup window is opening but not stay on the screen at the same time, it get closed. Why its happening?