HELP! My ATV back shock is messed up

[gio2008rider]

So I just took my ATV (the guy I bought it from said its a 125cc 2008 gio atv) for a short ride, and when I got back I noticed some black liquid leaking out from where the black shock is. When I checked it out, the shock was no longer connected to the axle thing, and a ton of this black stuff leaked out. What is the problem, how do I fix it, and what do I need to fix it? I know nothing about this stuff...