1. You can create an empty file called app_offline.htm and insert it into your /httpdocs/ folder.  This file is a trigger for ASP.net to load any content in the app_offline.htm file, and turn off the application pool.  If you wish to create a custom "site under construction" page, you can do so by placing it in the app_offline.htm file, and it will display this for you. 
   Please be aware that this option will disable your site completely, and you will not be able to make any changes directly through your CMS.
2. You can create an Index.htm and insert it into your /httpdocs/ folder.  This will "hide" your site behind this html page without turning off the application pool of the site.  You can then continue accessing the site by browsing directly to domain.com/default.aspx to make any required changes.

If you would like a basic "Under Construction" Index.htm to put in your httpdocs folder you can download the one below. To place it into the httpdocs folder of your site:

1. Log into your Control Panel.
   • In Plesk 9.x click on Domains > select the domain > File Manager > httpdocs link > Add New File >  Upload file  radio button > Choose File > locate and select it and click OK.
   • In Plesk 11.x click on your subscription > Websites & Domains > File Manager > httpdocs link > Add New File > Upload file  radio button > Choose File > locate and select it and click OK
   • In Plesk 12.x click on your subscription >Website at "/Httpdocs" > Upload file > Choose File > locate and select it and click OK.
2. It should be renamed to app_offline.htm.

Sample Index.htm