This happened to me after uninstalling a problematic extension. I received the error upon exiting Magento Connect. I found the solution.
PROBLEM: This sometimes occurs when there’s an error during installation or uninstalling an extension. Since Magento Connect usually puts the store into Maintenance Mode during the install or uninstall process...when there’s an error, the store gets stuck in maintenance mode, causing the “service temporarily unavailable” error.
SOLUTION: You must access your Magento installation via FTP and delete the file which Magento Connect generated during the install/uninstall process, called “maintenance.flag” It will be on the root level of your Magento folder. Don’t worry about this being deleted. Another will be generated (and appropriately removed) by the system, when you activate maintenance mode in the future.
If you have trouble accessing with your normal FTP, you can try something like FireFTP, which is a free plug-in for Firefox. It does a good job of processing files that my regular FTP client corrupts or doesn’t list.