I got this error message while trying to set up my on line site to use a different database from the default crap-tastic AspNetSqlProvider that takes 3 minutes to boot up with each web hit.
Here's the entire (incorrect) error message as IIS sends it.

Server Application Unavailable

The web application you are attempting to access on this web server is currently unavailable.  Please hit the "Refresh" button in your web browser to retry your request.
Administrator Note: An error message detailing the cause of this specific request failure can be found in the application event log of the web server. Please review this log entry to discover what caused this error to occur.

The first clue I got that something was Micro-softy was that there was no event logged in the event logger. I think I may have caused this problem when I logged in to the server with Remote Desktop and with Visual Web Developer at the same time, trying to make it select the correct database.

The Solution

1. go to the webserver (or remote-desktop in)
2. open IIS Manager (Microsoft Internet Information Services)
3. browse to the server and website that is broken
4. stop the website
5. open the site's properties page (right click the name of the site and pick "properties")
6. select the ASP.NET tab and change the .net version. Click Apply.
7. change the version back to the right (latest) version. Click Apply.
8. close the properties page and restart the website.