Some times Magento developers faced the issue of below :
Service Temporarily Unavailable The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.
You are not able to open even admin page and not even able to see any sort of errors on then page. So to solve this type of issues just follow below steps :
Magento 1.x : maintenance.flag file is in : magento root directory
Magento 2.x : maintenance.flag file is in : var folder
When Magento is performing some tasks, it temporarily creates this maintenance file. Magento checks for its existence and if it’s there will send users to the page you described.
It’s supposed to automatically delete this file when done processing whatever task it was doing, but I’ve experienced occasions where something went wrong and it failed to delete it.