How To Address Magento Error 503 : Service Unavailable?

The reason for this specific 503 error message on your Magento installation is that maintenance has been enabled. This problem may arise from the installation of extensions on your Magento website via the Magento Connect Manager, if you haven’t explicitly set the maintenance mode.

Accessing the root directory of your Magento installation is the only step required to solve this error. You have two options for doing this: using the File Manager service that comes with your cPanel account, or using FTP. Navigate to your Magento website’s directory and look for the file “maintenance.flag” there. You can restore normal access to your website by removing this file when you’ve located it.