Login to Magento server using SSH and create a file as below to place the entire Magent 2 shop in maintenace mode in the var folder.
Usually it is good to allow some IPs that belong you or your developers full access to the shop even during this maintenance mode. In order to accomplish this please add your IP addresses to this file like this in the var folder.
var/.maintenance.ip 126.96.36.199 188.8.131.52
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