WP Killswitch lets you non-destructively white out a WordPress website while this plugin is active (recommended as an mu-plugin).
To prevent anyone disabling or deleting this plugin it is recommended you add this to the mu-plugins directory via FTP.
When activated this plugin will white out the front end of the website and make it inaccessible for visitors. The admin panel is still visible at your admin URL. This plugin will also display a warning notification in the admin telling the client/administrator to contact their webmaster.
If you are using this plugin to hide a website while you work on fixing something for example – please note that by initiating a white out on the front end of a website, you are not deleting or turning off the website. The contents of the site will still be visible to search engines and text only browsers. Any user can still see the content by viewing the source code of the page.
Remote
If you need to activate WP Killswitch remotely for whatever reason, you can register your site at https://wpkillswitch.io
Screenshots
What the user sees on the front end
The warning message displayed in the admin