Today visiting one of the Magento sites I look after returned the 503 Server error, with the message that the server was down for maintenance or was out of resources. This was true for both the front end and the admin area.
Looking in my error logs returned this error message:
client denied by server configuration: MAGENTO_DIR/app/etc/local.xml
Google returned several possible solutions. But then I remembered I’d been in the back end, using Magento Connect Manager to upload a language pack. This had failed for some reason. It then dawned on me that Magento puts the store (CE 18.104.22.168) into maintenance mode when performing updates, etc. So, its probable that Magento did not return the site to live mode after the upload failed.
A quick look in the root showed that a file called “maintenance.flag” was there. Renaming the file meant both front and backend were now working as expected.