I was working on a custom HttpModule for SharePoint that was packaged in a Feature and Solution and (after hundreds of successful redeploys) I started seeing HTTP 503 errors and none of my sites would come up (even after an IISRESET and machine reboot).
After some digging, I noticed that the application pool for the SharePoint web app no longer started automatically when IIS came up.
Simply starting this manually fixed my problem and now the app pool starts automatically.
