Your Magento store shows “Service Temporarily Unavailable” page when it is switched into Maintenance mode. It can happen after extension installation via Magento Connect, scheduling a backup or performing any update/upgrade actions:

   

Solution

To disable “Service Temporarily Unavailable” page you need to delete maintenance.flag file:

• Login to your server and navigate to Magento root directory
• delete maintenance.flag file

$ cd /path/to/magento/root/public_html/
$ ls
LICENSE.html cron.php includes  mage LICENSE.txt  cron.sh  index.php media
LICENSE_AFL.txt downloader index.php.sample pkginfo RELEASE_NOTES.txt errors install.php shell api.php favicon.ico js skin app get.php lib var
 
$ rm maintenance.flag

For additional details on maintenance.flag and Maintenance mode please refer to this article.