Error!: SQLSTATE[HY000] [2002] No such file or directory

Fatal error: Uncaught Error: Call to a member function prepare() on null in /var/www/html/API/includes/lib/mysql.php:497 Stack trace: #0 /var/www/html/API/includes/lib/mysql.php(331): database->sqlPrepare('SELECT * FROM s...') #1 /var/www/html/sitemap/sitemap.php(13): database->sqlSelectAllSql('SELECT * FROM s...') #2 {main} thrown in /var/www/html/API/includes/lib/mysql.php on line 497