All major development or changes to a site should be performed on the development machine and once thoroughly tested then only be implemented to the live site.

There are chances when you have to make changes on live site directly. In this case you have to put the the live site on maintenance mode first and implement your changes.
Here how you can put you site on maintenance mode.

Visit the `Maintenance mode page` in the `Development` section, and select `Put site into maintenance mode`, provide a maintenance mode message to display if the default one is not suitable. once all setting saved get on with your work.

Be careful when working in maintenance mode because once you have logged out you are locked out too. This is because, by default, only administrative can work on the site while it is on maintenance mode.

If you logout, you are no more allowed to login the site and you will be treated as annonymous user for the site and if you do login other way around like I have mentioned below, then only the admin role can login.

Solution:

Drupal allows the login page to be accessed via http://localhost/drupal/user, where you will be able to login back as administrator.