My site was working, but now I only get the "Site is Down" screen
If you had a running site and without placing it in 'Mainenance Mode', it displays that screen and you are unable to log in as an admin, it means that your database is down or not found. You should notice the 'Database is currently Off-line!' message which is displayed if the database can not be reached.
If you feel this is only a transitory condition such as the database server being upgraded or temporarily offline, the system will work normally once the database connection is returned.
If however the condition doesn't change, your database settings inside /conf/config.php no longer match the database. You should first check the database server to ensure it is online, the database still exists and the spelling is correct, and the database user and database user password are still valid. Then you can either directly edit the /conf/config.php file to update the database settings or delete the /conf/config.php file and run the installation again.