[ultraman zoffy]

Pretty easy fix actually, but you'll need to find a swivel/hinge disc similar in size to the one that broke. If you do, use an x-acto, cut the disc out of the knee (only cut on ONE SIDE), boil the knee to soften the plastic, slip the broken disc out (you'll see the little plastic knub inside still attached on one side in the knee), slide the new disc on, dab a touch of super glue onto the little center knub, and pinch the knee around the disc while at the same swiveling the new disc back and forth so that it doesn't get glued in place. The center knub thing will glue back onto the side you cut it from, and the disc will still swivel and maintain it's articulation.
To get the snapped off piece out of it's socket, just boil it for a while, then use a small screwdriver to pry and pull it out.
Simple fix, and a good one to remember since this probably won't be the last time you experience it. Of course, you can always try going through "customer service", but good luck with that route depending on the company and if they even have any more units on hand to send you.