This particular 503 error message is shown because of enabled maintenance on your Magento installation. In case you have not specifically set the maintenance mode, this error might result because of the installation of extensions on your Magento website through the Magento Connect Manager.
There is an easy solution to removing this error, all you have to do is access the root directory of your Magento installation. You can either do this through FTP or you can directly make use of the File Manager service present in your cPanel account. Go to the directory of your Magento website and in this directory, search for the file – ‘maintenance.flag’. Once you locate this file, remove it and your website will be accessible normally again.