If you’re seeing the “Service Temporarily Unavailable” screen when you attempt to load your Magento store (there’s a somewhat related thread here on magentocommerce.com), the most likely cause is you’ve made some updates to Magento recently, and the maintenance.flag file in your Magento root directory wasn’t successfully removed by the Magento Connect tool.
Fixing the Service Temporarily Unavailable error in Magento
Nice and simple fix for this one! Remove the maintenance.flag file in the root directory of your Magento installation.
This can be triggered by running updates or installations of extensions in Magento Connect; the file is removed on successful completion of the upgrade or installation, so if it fails for any reasons, you’ll see this screen.