"Service unavailable" IIS 6.0 Error - Quick Resolution
Short story: If you get "service unavailable" error when trying to access a page hosted on IIS 6.0, login to the server on which IIS is running, open the command line and type in "iisreset " without quotes. That should bring the webpages back.
Long Story: I had increased the application pool threads to ten. You can do this by right-clicking on the Application Pool and going through the properties. Somehow, after the change, my website felt very sluggish. When I reset the Application Pool threads to 1, I got "service unavailable" error while trying to browse the site.
My first step of stopping & starting the IIS server did not work. I also have Tomcat ISAPI redirector for mapping the jsp pages to a separate server and once I thought isapi might be the culprit.
The easiest way would have been to restart IIS server box. But I was in a controlled environment and could not do it. Searching for ways to restart IIS server, I found the "iisreset" command, which did the trick.
Note: My environment was Windows 2003 Server, IIS 6.0 with Tomcat server on another box being mapped by Tomcat Connector.