My login page is not working

Hi, there!

I'm trying to create a login page and a registration page. The registration works perfectly, or I believe so, but the login not. When I introduce the username and the password, it brings me an error login message "login invalid" and I don't know why. I saw you here my code:

CONTROLLER (Pages.php)

PHP Code:

public function login(){
            $data['title'] = 'Sign In';

            $this->form_validation->set_rules('username', 'Username', 'required');
            $this->form_validation->set_rules('password', 'Password', 'required');

            if($this->form_validation->run() === FALSE){
                $this->load->view('templates/header');
                $this->load->view('users/login', $data);
                $this->load->view('templates/footer');
            } else {
                
                // Get username
                $username = $this->input->post('username');
                // Get and encrypt the password
                $password = md5($this->input->post('password'));

                // Login user
                $user_id = $this->user_model->login($username, $password);

                if($user_id){
                    // Create session
                    $user_data = array(
                        'user_id' => $user_id,
                        'username' => $username,
                        'logged_in' => true
                    );

                    $this->session->set_userdata($user_data);

                    // Set message
                    $this->session->set_flashdata('user_loggedin', 'You are now logged in');

                    redirect('servicioscolegiado');
                } else {
                    // Set message
                    $this->session->set_flashdata('login_failed', 'Login is invalid');

                    redirect('users/login');
                }        
            }
        } 


MODEL(User_model.php)

PHP Code:

public function login($username, $password){
            // Validate
            $this->db->where('username', $username);
            $this->db->where('password', $password);

            $result = $this->db->get('users');

            if($result->num_rows() == 1){
                return $result->row(0)->id;
            } else {
                return false;
            }
        } 


VIEW(login.php)

PHP Code:

    <?php echo form_open('users/login'); ?>
        <div class="row">
            <div class="col-md-4 col-md-offset-4">
                <div class="form-group">
                    <input type="text" name="username" class="form-control" placeholder="Enter Username" required autofocus>
                </div>
                <div class="form-group">
                    <input type="password" name="password" class="form-control" placeholder="Enter Password" required autofocus>
                </div>
                <button type="submit" class="btn btn-primary btn-block">Login</button>
            </div>
        </div>
    <?php echo form_close(); ?>

I'm seriously mad right now because I cannot see the problem and it's starting to be a little bit frustrating. Please any guide will be more than welcome.

Thank you so much and have a great day.