1. How to Fix : Service Temporarily Unavailable
When using the Magento Connect manager to install or delete extensions from the Magento Connect store, your store might display the 503 - Service Temporary Unavailable page. This often happens when the cache has not been flushed well or when the Magento core is neglected while leaving the Connect Manager. This problem is easy to solve.
To remove the maintenance mode, simply login to your FTP and locate a file called “maintenance.flag” in your Magento root directory and delete or rename this file.
After this, refresh your cache and your store will be up and running again!