message appear that mysite under maintenance ?
January 21st, 2020
or do some thing from the Cpanel make mysite Offline
Generally you don’t do that from cPanel. It depends how you plan to make your site, or have made your site.
If you’re using a CMS like Drupal, or WordPress (which I highly recommend you do), then this option is built into them. Otherwise, you just need to create a HTML/PHP file with some message and maybe graphics too; and you’d also probably want a .htaccess file! The only thing with no using a CMS is that there is no real way to restrict people from accessing the site you are working on unless you develop it all locally.