ModalPopupExtender.cs not found

One of the pages in my website has several forms that are supposed to open Modal popup panels. When I run it and click on one of the buttons, noting happens. IF I step through it in debug, when it gets to that point (panelAddress_ModalPopupExtender.Show()) I get the error "ModalPopupExtender.cs not found" and the debugger stops. THis did nt happen at one time and there are other pages that 'use the modal popup that work just fine, so I don't know why this code can't find the popup extender?  Here is the Design code:

<div style="width: 300px; float: left;"><%--User basic info frm--%><asp:Panel ID="pnlriderinfo" runat="server" CssClass="gainsPanel" Style="margin-top: 10px; margin-left: 15px; width: 250px; height: 265px;"><asp:FormView ID="FormView1" runat="server" DataKeyNames="riderid" DataSourceID="SqlDataSource1" Height="250px" Width="250px" CssClass="formstyle"><HeaderTemplate><asp:Image ID="imgicon" runat="server" ImageUrl="~/Images/Site/bdicon.png" /><asp:Label ID="lblbdtitle" runat="server" Text="Your Big Dog Profile" Font-Bold="true" Font-Size="Medium" /><hr /></HeaderTemplate><ItemTemplate><table><tr><td class="tdrt_100"><asp:Label ID="lblfullname" runat="server" Text="Full&nbsp;Name:" CssClass="labelstd" /></td><td class="tdlt_150"><asp:Label ID="TitleLabel" runat="server" Text='<%# getName() %>' CssClass="formstyle" /></td></tr><tr><td class="tdrt_100"><asp:Label ID="lbldispname" runat="server" Text="Display&nbsp;Name:" CssClass="labelstd" /></td><td class="tdlt_150"><asp:Label ID="Display_NameLabel" runat="server" Text='<%# Bind("[dname]") %>' CssClass="formstyle" /></td></tr><tr><td class="tdrt_100"><asp:Label ID="Labelbd" runat="server" Text="Birth&nbsp;Date:" CssClass="labelstd" /></td><td class="tdlt_150"><asp:Label ID="BirthdateLabel" runat="server" Text='<%# Bind("dob", "{0:d}") %>' CssClass="formstyle" /></td></tr><tr><td class="tdrt_100"><asp:Label ID="Labelgewn" runat="server" Text="Gender:" CssClass="formstyle" /></td><td class="tdlt_150"><asp:Label ID="GenderLabel" runat="server" Text='<%# getSex(Eval("gender").ToString()) %>' CssClass="formstyle" /></td></tr><tr><td class="tdlt_100"><asp:Label ID="Labelcom" runat="server" Text="Comment:" CssClass="labelstd" /></td><td></td></tr><tr><td colspan="2"><asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("comment") %>' CssClass="formstyle" BorderStyle="Solid" BorderWidth="1px" BorderColor="LightSteelBlue" TextMode="MultiLine" Rows="4" Width="200px" /></td></tr></table><br /><asp:Button ID="bDetail" runat="server" Text="Edit" OnClick="bDetail_Click" ToolTip="Press to Edit this information" /></ItemTemplate></asp:FormView></asp:Panel><%--End basic info--%><%-- Base info Modal panel --%><%--End of user info updatepanel--%><asp:SqlDataSource ID="sqlTitle" runat="server" ConnectionString="<%$ ConnectionStrings:BDUCConnectionString %>" SelectCommand="titlefetch" SelectCommandType="StoredProcedure" /><%--rider popup modal start--%><ajaxToolkit:ModalPopupExtender ID="pnlUserInfo_ModalPopupExtender" runat="server" Enabled="True" BackgroundCssClass="modalBackground"
                TargetControlID="Button1" PopupControlID="pnlUserInfo" OkControlID="UpdateButton" CancelControlID="UpdateCancelButton" /><asp:Button ID="Button1" runat="server" Text="Button" Style="display: none" /><%--popup end--%><asp:UpdatePanel runat="server" ID="updatescale" ChildrenAsTriggers="false" UpdateMode="Conditional"><ContentTemplate><asp:Panel ID="Panel5" runat="server" Height="71px" CssClass="gainsPanel" Style="margin-top: 0px; margin-left: 15px; width: 250px;"><div style="float: left; margin-left: inherit;"><asp:Label ID="Label3" runat="server" Text="Preferred Scale:" CssClass="labelstd"></asp:Label></div><div style="float: left;"><asp:RadioButtonList ID="rblscale" runat="server" RepeatDirection="Horizontal"
                                AutoPostBack="True" OnSelectedIndexChanged="rblscale_SelectedIndexChanged"
                                Style="font-family: sans-serif; font-size: .8em; font-weight: 200;"><asp:ListItem Value="I">US Std (Imperial)</asp:ListItem><asp:ListItem Value="M">Metric</asp:ListItem></asp:RadioButtonList></div></asp:Panel></ContentTemplate></asp:UpdatePanel></div><%--SECOND COlumn--%><div id="d2" style="float: left; width: 270px; margin-left: 15px;"><asp:Panel ID="Panel3" runat="server" CssClass="gainsPanel" Style="margin-left: 0px; width: 265px;"><asp:FormView ID="FormView2" runat="server" DataSourceID="dsAddress" Height="265px" Width="265px"><HeaderTemplate><asp:Image ID="Image2" runat="server" ImageUrl="~/Images/Icons/doghse.png" /><strong>Location Information</strong><hr style="width: 187px; margin: auto;" /></HeaderTemplate><ItemTemplate><%--<asp:Label ID="addressIDLabel" runat="server" Text='<%# Eval("addressID") %>' />--%><asp:HiddenField ID="hfaddressid" runat="server" Value='<%# Eval("addressID") %>' /><table><tr><td class="tdrt_100"><asp:Label ID="Labeladd1" runat="server" Text="Address:" CssClass="labelstd" /></td><td class="tdlt_150"><asp:Label ID="address1Label" runat="server" Text='<%# Bind("address1") %>' CssClass="txlt_150" /></td></tr><tr><td class="tdrt_100"><asp:Label ID="Labeladd" runat="server" Text="Address&nbsp;2:" CssClass="labelstd" /></td><td><asp:Label ID="address2Label" runat="server" Text='<%# Bind("address2") %>' /></td></tr><tr><td class="tdrt_100"><asp:Label ID="Label2" CssClass="labelstd" runat="server" Text="City:" /></td><td class="tdlt_150"><asp:Label ID="cityLabel" runat="server" Text='<%# Bind("city") %>' /></td></tr><tr><td class="tdrt_100"><asp:Label ID="Labelst" CssClass="labelstd" runat="server" Text="State:" /></td><td class="tdlt_150"><asp:Label ID="stateidLabel" runat="server" Text='<%# getState(Eval("stateid").ToString()) %>' /></td></tr><tr><td class="tdrt_100"><asp:Label ID="Label11" CssClass="labelstd" runat="server" Text="Postal&nbsp;Code:" /></td><td class="tdlt_150"><asp:Label ID="postalcodeLabel" runat="server" Text='<%# Bind("postalcode") %>' /></td></tr><td class="tdrt_100"><asp:Label ID="Labelph" CssClass="labelstd" runat="server" Text="Phone:" /></td><td><asp:Label ID="phoneLabel" runat="server" Text='<%# Bind("phone") %>' /></td></tr><tr><td class="tdrt_100"><asp:Label ID="Labelcntryr" CssClass="labelstd" runat="server" Text="Country:" /></td><td class="tdlt_150"><asp:Label ID="countryLabel" runat="server" Text='<%# getCountry(Eval("iso").ToString()) %>' /></td></tr></table><asp:Button ID="btneditadd" runat="server" OnClick="btneditadd_Click" Text="Edit" /><br /></ItemTemplate></asp:FormView></asp:Panel>

And the C#

   protected void bDetail_Click(object sender, EventArgs e)
    {
        pnlUserInfo_ModalPopupExtender.Show();
    }

The method is   hit when debugging, but you get the error. I have looked at quite a few posts about similar issues but have not been able to fix this.