Winword RPC server is unavailable. (Exception from HRESULT: 0x800706BA)

private Word.ApplicationClass WordApp = new Word.ApplicationClass();

public void OpenDocument(object DocName)
       {
           try
           {

               WordApp.Visible = true;
               object optional = System.Type.Missing;
               object missing = System.Reflection.Missing.Value;
               object isVisible = true;
               Word.Document aDoc = WordApp.Documents.Open(ref DocName, ref       optional, ref optional,
                   ref optional, ref optional, ref optional, ref optional, ref optional, ref optional,
                   ref optional, ref optional, ref optional);
               aDoc.Activate();
           }

           catch (Exception e)
           {
               MessageBox.Show(e.Message);
           }
       }
private void listBox1_DoubleClick(object sender, EventArgs e)
       {
           try
           {
               string DocName;
               DocName = txtPath.Text.ToString();
               OpenDocument(DocName);
           }
           catch (Exception er1)
           {
               MessageBox.Show("" + er1.Message);
           }
       }