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 18.104.22.168 22.214.171.124
Schogini assures 24x7 - 6 hour turn around ticket response time and 24 hour Email response time (email@example.com)