Your web pages are served by an application pool. If you disable/stop the application pool, and anyone tries to browse the application, you will get a Service Unavailable. It can happen due to multiple reasons…
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Your application may have crashed [check the event viewer and see if you can find event logs in your Application/System log]
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Your application may be crashing very frequently. If an app pool crashes for 5 times in 5 minutes [check your application pool settings for rapid fail], your application pool is disabled by IIS and you will end up getting this message.
In either case, the issue is that your worker process is failing and you should troubleshoot it from crash perspective.
What is a Crash (technically)… in ASP.NET and what to do if it happens?