This error indicates that your application pool has crashed or is throttled.
Every web hotel is isolated in its own application pool. When the application pool crashes is this due to that something has happened with in the application pool. This could be code that was executed wrongly, SQL connections that did not get closed and so on. Another reason could be that your web hotel is out of resources or space.
The problem can often be solved by restarting the application pool (stop/start) through IIS Manager.
Article from the support category: ASP & ASP.NET