LDAP Authentication .net giving exception "The server is not operational"

private void btnLogin_Click(object sender, EventArgs e)
      {
          string strUsername, strPassword, strDomain;
          strUsername = txtUsername.Text;
          strPassword = txtPassword.Text;
          strDomain = txtDomain.Text;
          if (strUsername == "")
              MessageBox.Show("Enter Username");
          else if (strPassword == "")
              MessageBox.Show("Enter Password");
          else if (strDomain == "")
              MessageBox.Show("Enter Domain");
          else
          {
              strDomain = "LDAP://" + strDomain;
              using (DirectoryEntry deDirEntry = new DirectoryEntry(strDomain,
                                                        strUsername,
                                                         strPassword, AuthenticationTypes.Secure))
              {
                  // if user is verified then it will welcome then
                  try
                  {
                      MessageBox.Show("Welcome to '" + deDirEntry.Name + "'");

                      // TODO: add your specific tasks here
                  }
                  catch (Exception exp)
                  {
                      MessageBox.Show("Sorry, unable to verify your information"+exp);
                  }
              }
          }
      }

System.Runtime.InteropServices.COMException (0x8007203A): The server is not operational.

   at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)
   at System.DirectoryServices.DirectoryEntry.Bind()
   at System.DirectoryServices.DirectoryEntry.get_Name()
   at ldap.frmLdap.btnLogin_Click(Object sender, EventArgs e) in C:\Documents and Settings\divyar.DNISC\my documents\visual studio 2010\Projects\ldap\ldap\frmLdap.cs:line 41