0 Magento:Service Temporarily Unavailable The server is temporarily unable to service your request...

There are two main reasons why you may get this error.

1. There is an actual problem with the server, resulting in the 503 error response. In this case, you should contact your hosting provider.

2. Your store has been put in maintenance mode. If Magento sees a file named maintenance.flag in the root directory, it will automatically redirect all requests to the default 503 error page. 

SOLUTION:
delete the following file from your /magento/ installation

maintenance.flag

e.g. rm ./magento/maintenance.flag