Knowledgebase
How to fix the error 503: "Service Temporarily Unavailable" in Magento
When you use Magento Connect Manager to install/uninstall extensions, upgrade Magento, etc. the site is put into maintenance mode by default. So even after you're done with the Connect Manager if somebody visits the frontend of your site (or the backend too), they will see the error message Service Temporarily Unavailable.
The solution to this problem is quite simple. To disable the maintenance mode you need to delete a file called maintenance.flag from the root Magento directory on your hosting account. So, for example, if your application is installed directly in the public_html folder on your Magento hosting account, meaning the frontend is accessible at yourdomain.com, then inside public_html find the file maintenance.flag and delete it. You can do that from the Files section of the Pixie control panel, or you can use an FTP client (e.g. FileZilla).