How do I set maintenance mode on?

WP-CLI's command wp maintenance-mode creates .maintanance file to the root of the WordPress installation.

$ wp maintenance-mode activate
Enabling Maintenance mode...
Success: Activated Maintenance mode.

The file sets WordPress installation to maintenance mode, during which you can not access the WordPress page through a browser.

You can end the maintenance mode by using command wp maintenance-mode deactivate. This removes the .maintenance file.

$ wp maintenance-mode deactivate
Disabling Maintenance mode...
Success: Deactivated Maintenance mode.

WordPress documentation on wp maintenance-mode

You can also set the page to a maintenance mode using a maintenance plugin.

Maintenance is easy to easy and efficient plugin. After installing the plug-in you can activate a maintenance mode. From settings you can adjust the look of the maintenance mode. Once in the maintenance mode, in the right of the screen there is a lock icon that you can use to login in to the website.

If necessary, empty the WordPress cache to enter maintenance mode immediately.