我使用 Composer 处理一个项目。但是当我运行索引文件时，系统显示以下错误，
Warning: require_once(vendor/autoload.php): failed to open stream: No such file or directory in D:\xampp\htdocs\instagram_php\index.php on line 5
Fatal error: require_once(): Failed opening required 'vendor/autoload.php' (include_path='.;D:\xampp\php\PEAR') in D:\xampp\htdocs\instagram_php\index.php on line 5
我已经从https://getcomposer.org/安装了作曲家。我做错了什么？