Here is a sample database.php in case you need to rebuild it manually. Make sure to fill in the areas between the quotes as well as the cachedir path.
<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
$active_group = 'expressionengine';
$active_record = TRUE;
$db['expressionengine']['hostname'] = "";
$db['expressionengine']['username'] = "";
$db['expressionengine']['password'] = "";
$db['expressionengine']['database'] = "";
$db['expressionengine']['dbdriver'] = "mysql";
$db['expressionengine']['dbprefix'] = "exp_";
$db['expressionengine']['pconnect'] = FALSE;
$db['expressionengine']['swap_pre'] = "exp_";
$db['expressionengine']['db_debug'] = TRUE;
$db['expressionengine']['cache_on'] = FALSE;
$db['expressionengine']['autoinit'] = FALSE;
$db['expressionengine']['char_set'] = "utf8";
$db['expressionengine']['dbcollat'] = "utf8_general_ci";
$db['expressionengine']['cachedir'] = "/path/to/system/expressionengine/cache/db_cache/";
/* End of file database.php */
/* Location: ./system/expressionengine/config/database.php */