Upgrading from CI 2.2.1 to CI 3.1.3(Though the first page opens up but login doesn't)

I have upgraded from CI 2.2.1 to CI 3.1.3 using the tutorial but the first web page shows up without any error but the login doesn't work. I am using files driver 
$config['sess_driver'] = 'files';
$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 86400;
$config['sess_save_path'] = APPPATH.'ci_sessions/';
$config['sess_match_ip'] = FALSE;
$config['sess_time_to_update'] = 86400;
$config['sess_regenerate_destroy'] = FALSE;

I am newbie to codeigniter. Any help related to this would be really appreciated.