Today I got this error on my IIS website:
HTTP Error 503. The service is unavailable.
After a bit of troubleshooting, I found that it was because the Application Pool used by the website was in the stopped state. I started the Application Pool and the website started working. If you can’t start the Application Pool, try going to the advanced settings of the application pool and editing the Application Pool Identity (ensuring to use an account that has permission to the directory containing the website’s files) then restarting.