Sometimes you need to take your site offline, for example, when you do monthly maintenance or upgrading the site. You can turn the site off and set a message for your site audience.

Step 1: Go to Admin > Configuration > Site maintenance.

Step 2: Choose Online or Offline mode, type some messages for your site readers and then Save.

Master Drupal in 7 hours - Ongoing administration - Site maintenance

It is useful to know how to log in once you have turned your site off-line for maintenance.