If you want to shut down a site for maintenance and let your visitors know that it is temporarily unavailable, you can suspend the website so that it will not open in browsers (Websites & Domains > domain name > the Suspend link). Visitors will be redirected with the search engine friendly 503 HTTP code (Service Unavailable) to the "503 Service Unavailable" error page. The site's search engine rankings will not be affected, and the hosting services such as mail will still be available and manageable by means of Plesk.
You can customize the error page using the link Edit error page in Customer Panel > Websites & Domains > domain name.
The link Edit error page is displayed only if your hosting plan provides the option to customize web server error documents (the Custom error documents is On in the domain settings in Websites & Domains > domain name > Hosting Settings).