404 error, need HELP!

[eluser]Keloo[/eluser]
I'm currently working on a project and suddenly I'm getting this "404 page not found" error, when I'm trying to logout or to access the cart page.
Here is part of the code :

Code:

function validate_credentials()
  {  
    $this->load->model('users_model');
    $query = $this->users_model->validate();
    
    if($query) // if the user's credentials validated...
    {
      $data = array(
        'username' => $this->input->post('username'),
        'is_logged_in' => true
      );
      $this->session->set_userdata($data);
      redirect('site/index');
    }
    else // incorrect username or password
    {
      $this->index();
    }
  }

function logout()
  {
    $this->session->sess_destroy();
    $this->index();
    redirect('site/index');
  }
function cart() {
        $data['main_content'] = 'cart';
         $this->load->view('includes/template',$data);
        $this->is_logged_in();
  }

Everything was fine until a few days ago, and now I cannot logout of the site, or access the cart section.
I think something is wrong with the sessions, but I'm not sure. I hope someone can give me a hint.