Could happen that after upgrading magento to a new version instead of admin/frontend all you see is error page with following text:
Service Temporarily Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.
Probably your store is in the maintenance mode, you can disable it by deleting following file: maintenance.flag which is found in your magento root folder. If there isn’t such file, you should search for the source of error elsewhere, first of all in php error_log file.