If you need to temporarily close your site to users for technical work, you can easily do so. Access the Admin Panel of your site and enable the "Make the site disabled" option in Configuration - My Store - Other Settings:
Now your site will be closed to users (but it will remain accessible to the administrator), and they will see the following message:
You also have the opportunity to edit this message. Simply go to Content - Pages and edit the "Under maintenance" article:
Fill in the text in all the languages available on your site and click "Save":