Replacement part for Bonnie Trek bike?

I have a 'Bonnie' pink Trek cruiser. Awhile back I seemed to have lost something on the back tire, its a screw that connects a long silver... whatever you may call it, to the tire bar. I am not really a handy woman so I am not sure what to look for frankly. Is there anyway someone can help me find a link to replacement parts or find what kind of screws my model uses? I don't want to have to bring it in just to find a screw, also that's a bit silly so. All I know in my stupid talk is its a silver screw that isnt longer than 1/2 an inch?