Issue

Each web site runs in it's own IIS Application Pool for script isolation. If your IIS Application Pool has stopped or crashed you will see a 'Service Unavailable' notice when browsing your web site.

Solution

Start the IIS Application Pool using Plesk.

To start the Application Pool follow these steps:

1. Login to Plesk Server Administrator.
2. Select the domain name you are having the problem with.
3. Click on the IIS Application Pool button.
   
   
4. Click on the Start button.
   
   
   

After a few seconds the page will reload and the Work Process will have been started. Your web site should now be back online.