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
404 OOPS!
page Not found!
"Sorry, it appears the page you were looking for doesn't exist anymore or might have been moved.
Please try your luck again."
This is Joomla! Article, You can change text and titile in Article Manager, click the Content → Article Manager menu item and find article Error 404.
Search module (position: error-search) is displayed by plugin - Content - Load Modules which allow users to publish modules in Joomla articles.
If you want select other article you can change article ID in /templates/jm-template/error.php
You can make your Joomla! website unavailable to visitors until selected date. To enable this option login to Administrator back-end.
1. Click the Extensions → Template Manager → JM-Template-Name → Basic Settings menu item
2. Find field Coming Soon and click Enable button.
3. Next set date in filed Coming Soon Date.
Click the Save button to implement the new settings.
Title and description you can change in Extensions → Module Manager. Find module Our site is coming soon on position coming-soon. You can also publish different type of module if you like.
Coming Soon page can be easily customised in file - /templates/jm-template-name/tpl/comingsoon.php