Account hijacked, turns out okay

So I abandon league for a couple years because I am bad. Now I find myself picking it back up. So I try to log into one of my accounts, this one specifically, and none of my passwords match. I reset it, log in, and see that someone's been using it for YEARS, bought a bunch of champs with IP, got up to Plat V, got demoted after playing a few unlucky games. I kind of feel bad for taking it away from this kid now. Still, wonder how this happened. What should I do about this?