How to use Maintenance Mode in Moodle | Moodle and Web 2.0 | Scoop.it
Updates, upgrades, new features, new development… these are just a few of the many tasks that need to be done in order to keep your Moodle experience fresh and compelling.

It is always good practice to schedule routine maintenance and upgrade events during times when the fewest (or no) users will be accessing the LMS (this, obviously, isn’t possible in the rare emergency event). The best way to ensure users are not on the system during these periods is through a high quality communication strategy.

During the maintenance period, it is also a best practice to set your site into Maintenance Mode, to prevent anyone except administrators from accessing the LMS. From Moodle.org:

An administrator can put the site into maintenance mode in Administration > Site administration > Server > Maintenance mode.

Users accessing the site when it is in maintenance mode will continue to see a normal homepage, but they will receive a notice that the site is currently under maintenance if they try to access course materials. (Note: this message to users can be customized to best fit your site and style).

Via Miloš Bajčetić