you may have renamed your user control but haven't updated ascx file references accordingly. for example
<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="PreOrderControl.ascx.cs" Inherits="MyApp.PreOrderControl" %>
and your PreOrderControl.ascx.cs file would be like below,
namespace MyApp
{
public partial class PreOrderControl : System.Web.UI.UserControl
{
note the
CodeBehind
in ascx file and the
Inherits
properties, they should match with the file name and the class name with your namespace