Database error in localhost

i downloaded the website from server to localhost and getting the following error

PHP Code:

A Database Error Occurred
Unable to connect to your database server using the provided settings.

Filename: controllers/Home.php

Line Number: 24 


This is whats contained in the Home.php file

PHP Code:

    *         http://example.com/index.php/welcome
     *    - or -
     *         http://example.com/index.php/welcome/index
     *    - or -
     * Since this controller is set as the default controller in
     * config/routes.php, it's displayed at http://example.com/
     *
     * So any other public methods not prefixed with an underscore will
     * map to /index.php/welcome/<method_name>
     * @see https://codeigniter.com/user_guide/general/urls.html
     */


        public function __construct(){
                parent::__construct();
                 $this->load->helper('form');
                 $this->load->library('user_agent');
                 $this->load->library('paypal_lib');
                 $this->load->model('Ad_model', 'Ad_user');
                 $this->load->model('User_model', 'user_model');
              $this->load->library('VRClient');
                 $this->load->library('form_validation');
           } 


How do i resolve this?