AppOffline error with MVC 2 app

I have installed my ASP.NET MVC 2 app to our QA servers. I set up a virtual directory and an app pool. Both have been started. But when I got the URL I get the error Service Unavailable. Not sure what this is? If I go to the log files in C:\WINDOWS\system32\LogFiles\HTTPERR then the only error that I see is:

2010-11-0开发者_开发技巧9 11:59:00 [IP removed] 10.58.10.79 80 HTTP/1.1 GET / 503 1118619102 AppOffline HBFGA

IIS is running on Windows 2003.

Please can someone help me?

This indicates that there's some problem with the application pool. You may take a look at the server Event Log for further details as to why it has failed.