Assuming that Magento is not actually in maintenance mode (i.e. downloader is not being used) then the following error is the result of a lock file not being removed, probably as the result of an ungraceful exit from the downloader:

Service Temporarily Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

To take Magento out of maintenance mode, just delete the file:

maintenance.flag

from the Magento root.