problem when loading a document

When I Build i try to load a project document I get the following message:

An error occured while loading the document. Fix the error, then try loading the document again. The error message follows:
 The designer could not be shown for this file because none of the classeswithin it can be designed. The designer inspected the following classes in the file:
     PrinterGUI2D--The base class 'RL.NDM.NDMUI.HostGUI2D' could nt be loaded. Ensure the assembly has been referenced or built if it is part of the project.


What d you advice. Below is file PrinterGUI2D.cs

namespace RL.NDM.NDMUI {
    using System;
    using System.Collections;
    using System.Core;
    using System.ComponentModel;
    using System.Drawing;
      using System.Diagnostics;
      using RL.NDM;
    using System.Windows.Forms;
   
      /// <summary>
      /// A PrinterGUI2D is an extension of a HostGUI2D, with graphics customised for a printer
      /// </summary>
    public class PrinterGUI2D : HostGUI2D {
        private System.ComponentModel.IContainer components;

            public PrinterGUI2D() : base() {
                  InitializeComponent();
            }

        public PrinterGUI2D(Host objHost) : base(objHost) {
              // This call is required by the Windows Form Designer.
              InitializeComponent();
            }

            // Form overrides dispose to clean up the component list.
        public override void Dispose() {
              base.Dispose();
              components.Dispose();
        }

        /// <summary>
        ///    Required method for Designer support - do not modify
        ///    the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
            {
                  System.Resources.ResourceManager resources = new System.Resources.ResourceManager (typeof(PrinterGUI2D));
                  this.components = new System.ComponentModel.Container ();
                  //@this.TrayLargeIcon = false;
                  //@this.TrayAutoArrange = true;
                  //@this.TrayHeight = 90;
                  lblHostname.Visible = true;
                  picIcon.Visible = true;
                  picIcon.Image = (System.Drawing.Image) resources.GetObject ("picIcon.Image");
                  ilstImages.ImageStream = (System.Windows.Forms.ImageListStreamer) resources.GetObject ("ilstImages.ImageStream");
            }

            public override void Refresh() {
                  base.Refresh();
            }
                                                                    
      }
}