Jammed Nikon camera lens?

I have a Nikon D50 and a minute ago I went to focus my 18-55mm lens and now it has jammed. It hasn't done this before and I can't get it unstuck. I just looked into the lens and there's a black plastic cylinder which seems to just be rattling around. Does anybody know what could have happened?

No but it'll take a professional to repair it. The lens, depending on its age, may not be worth the cost of repair. If not, just replace it.