Posting in the Magento forums has been disabled pending the implementation of a new and improved forum solution which should better serve the community.
For new questions please post at magento.stackexchange.com, the community-run support site for the Magento community. We will be providing updates on the new forum solution soon. For questions or concerns please email firstname.lastname@example.org.
As Monty22 suggested, this happens when Magento sees a file named maintenance.flag in the root directory. It will then show the content of errors/503.php.
When installing something through Magento Connect or installing upgrades, Magento automatically puts the store into Maintenance mode but if the installation fails for some reason the maintenance.flag file is not removed.
Just rename or remove the file and your site should be back up.