To make your WordPress website unavailable to visitors, replacing it with a simple message, do this:

1. Log in to the Administrator dashboard.
2. Click on the Theme Settings button in the admin sidebar.
3. Click in the top right corner on the Extras button and select Platform Settings.
4. Find where it says Offline Mode and select the Enabled radio button.
5. Optional: Change the Offline Message to give your visitors some explanation about why your website is unavailable.
6. Click the Save Changes button to implement the new settings.
7. You should see a message confirming the settings have been changed.