Yea that brass thing fell out on me too on both my engines, You can kinda just put it back on the sparkplug and jam the boot back on, but it never really goes back togethor right and falls apart every time you remove it.. theres also a spring behind the brass thing.
Really, your better off buying a proper boot and a normal sparkplug with the little nub on the end. They stay togethor forever, Insted of.. a week.