There is a possibility to disable access to the front of your site.

How to enable the offline page?
Step 1: Go to "System → Global Configuration → Site".
Step 2: Click on "Yes" for "Site Offline" option.
Step 3: Fill in a text for "Offline Message" to give your visitors some explanation about why your website is unavailable.
Step 4: Click on "Save" to save your settings.

How to customize the offline page?
You need to edit the following file: /templates/jm-template/offline.php

This is a Joomla! article displayed as custom 404 page.

How to modify this page?
Step 1: Go to Content → Article Manager.
Step 2: Find and edit the "Error 404" article.

How is the search module displayed?
It is displayed using the "Content - Load Modules" plugin. It allows to display a module in a Joomla article.
The module is published on the following position: error404-search

How to display a different article as custom 404 page?
If you want to display other article, you need to change the article ID in the following location: /templates/jm-template/error.php