We'll create fresh WordPress site with Under Construction page display for certain page is in under maintenance. installed. You have 20 minutes to test the plugin after that site we'll be deleted.
Add a maintenance page to your blog that lets visitors know your certain page is down for maintenance, add a coming soon page for a new website. User with admin rights gets full access to the blog including the front end.
Activate the plugin and your blog is in maintenance-mode, works and only registered users with enough rights can see the front end.