Without any reason, IIS may stop responding and eventually, fail to start manually or automatically.
You may notice the following message in the EventViewer :
Cannot start a new logon session with an ID that is already in use.
This is because a core dll file (qmgr.dll, which is Background Intelligent Transfer Service) is no longer registered on the system. Using the command prompt, you need to do the following and then reboot the machine :
regsvr32 qmgr.dll