title: ASP.Net Button not raising postback
body: <p>I've a simple page in one of our web applications, which has the following markup:</p>
  
  <pre><code>    &lt;%@ Page Language="C#" AutoEventWireup="true" CodeBehind="NewUpload.aspx.cs" Inherits="Mass_Upload.NewUpload" MasterPageFile="~/Master" Title="Document Mass Upload" %&gt;
  &lt;asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server"&gt;
      &lt;link rel="Stylesheet" type="text/css" href="./../CSS/ScrollingTable.css" /&gt;
      &lt;script type="text/javascript" src="../Help/HelpPopup.js" /&gt;
  &lt;/asp:Content&gt;
  &lt;asp:Content ID="Content2" ContentPlaceHolderID="CenterH1" runat="server"&gt;
      Document Mass Upload &lt;a href="javascript:loadHelpVid(5)"&gt;&lt;img style="Border:None;" src="../Help/help_icon.gif" /&gt;&lt;/a&gt;
  &lt;/asp:Content&gt;
  &lt;asp:Content ID="Content3" ContentPlaceHolderID="CenterBody" runat="server"&gt;
      &lt;h3&gt;Add New Upload&lt;/h3&gt;
      &lt;table class="list"&gt;
          &lt;tr&gt;
              &lt;td class="label" style="text-align:right;"&gt;Local File:&lt;/td&gt;
              &lt;td class="normal"&gt;
                  &lt;asp:FileUpload ID="fuFilename" runat="server" Width="405" /&gt;
                  &lt;asp:RequiredFieldValidator ID="RequiredFieldValidator1" Text="*"
                              ErrorMessage="A file to upload is required"
                              Display="Dynamic"
                              ControlToValidate="fuFilename"
                              ValidationGroup="DocumentUpload"
                              runat="server" /&gt;
              &lt;/td&gt;
          &lt;/tr&gt;
          &lt;tr&gt;
              &lt;td class="label" style="text-align:right;"&gt;Document Description:&lt;/td&gt;
              &lt;td class="normal"&gt;
                  &lt;asp:TextBox ID="txtDescription" runat="server" Width="405" MaxLength="50" /&gt;
                  &lt;asp:RequiredFieldValidator ID="RequiredFieldValidator3" Text="*"
                              ErrorMessage="Document Description is a required field"
                              Display="Dynamic"
                              ControlToValidate="txtDescription"
                              ValidationGroup="DocumentUpload"
                              runat="server" /&gt;
              &lt;/td&gt;
          &lt;/tr&gt;
          &lt;tr&gt;
              &lt;td class="label" style="text-align:right;"&gt;Document Type:&lt;/td&gt;
              &lt;td class="normal"&gt;
                  &lt;asp:DropDownList ID="ddDocType" runat="server" Width="405"/&gt;
                  &lt;asp:RequiredFieldValidator ID="RequiredFieldValidator2" Text="*"
                              ErrorMessage="Document Type is a required field"
                              Display="Dynamic"
                              ControlToValidate="ddDocType"
                              ValidationGroup="DocumentUpload"
                              runat="server" /&gt;
              &lt;/td&gt;
          &lt;/tr&gt;
          &lt;tr&gt;
              &lt;td class="label" style="vertical-align:top;text-align:right;"&gt;Customer Types:&lt;/td&gt;
              &lt;td class="normal"&gt;
                  &lt;asp:Label ID="lblSingleCustomer" Text="Specific Code:" runat="server" /&gt;&lt;asp:TextBox ID="txtSingleCustomer" runat="server" Width="100px" /&gt;&lt;br /&gt;
                  &lt;asp:CheckBoxList ID="cblCustomerTypes" runat="server" Width="405px" RepeatDirection="Horizontal" RepeatColumns="5" RepeatLayout="Table" CellPadding="10" CellSpacing="0" /&gt;
              &lt;/td&gt;
          &lt;/tr&gt;
          &lt;tr&gt;
              &lt;td class="normal" colspan="2"&gt;&amp;nbsp;&lt;/td&gt;
          &lt;/tr&gt;
          &lt;tr&gt;
              &lt;td class="normal" colspan="2"&gt;&lt;asp:Label ID="lblError" runat="server" Text="" ForeColor="Red"/&gt;&lt;/td&gt;
          &lt;/tr&gt;
          &lt;tr&gt;
              &lt;td class="normal" colspan="2"&gt;
                  &lt;asp:Button ID="btnCancel" runat="server" Text="Cancel" OnClick="BtnCancel_Click" CssClass="medium" /&gt;
                  &lt;asp:Button ID="btnUpload" runat="server" Text="Upload" OnClick="BtnUpload_Click" CssClass="medium" /&gt;
              &lt;/td&gt;
          &lt;/tr&gt;
      &lt;/table&gt;
  &lt;/asp:Content&gt;
  </code></pre>
  
  <p>It USED to work fine, but now, and without apparent change to code/design, both the "Upload" and "Cancel" buttons no longer work.</p>
  
  <p>Putting a breakpoint in the codebehind's Page_Load() method shows that it is only called when the page is initially loaded, and not when the button is pressed.
  Similarly, putting a breakpoint in the "BtnUpload_Click" event shows it is never called.</p>
  
  <p>This is now not working both on my own development machine AND on the client's server (both when browsing to the servers page from my machine AND from the server itself).</p>
  
  <p>It's important to stress that, between this working and it now not working, I am 90% sure nothing has changed in regards to the code.</p>
  
  <p>Any help would be greatly appreciated, as the customer is rightly anxious - and i'm clueless as to what's causing it!</p>
  
  <hr>
  
  <p><strong>EDIT #1</strong></p>
  
  <p>Here's the codebehind for one of the buttons:</p>
  
  <pre><code>protected void BtnUpload_Click(object sender, EventArgs e)
      {
          if (DataAccess.CheckIfMassUploadAlreadyExists(fuFilename.FileName))
          {
              lblError.Text = "A file with the specified name already exists within the system.";
              return;
          }
          else
          {
              try
              {
                  UploadFile();
              }
              catch(Exception ex)
              {
                  lblError.Text = ex.Message;// +"\nUsername:" + System.Web.HttpContext.Current.User.Identity.Name;
                  return;
              }
          }
      }
  </code></pre>
  
  <p>.</p>