How can I reattach a chandelier light fixture to the chain?
You need to just install a new loop. But, fortunately, the fix is pretty cheap and simple. I don't think glue is going to do it.

You'll want to shop around for one that matches the color, size, and so forth, but here's a ring on Home Depot's website as an example, and another one from Menard's website. You might want to take the chandelier apart first and take the broken ring with you test the color/fit at a store if you can.

First off, cut the power to the light. 

Then, at the top of the chain, there's likely to be a metal plate (usually with a similar ring) and/or thumbscrews that hold it to the ceiling box. Unscrew the ring or thumbscrew and lower the plate from the ceiling, exposing the wires. Untwist the wire nuts and take the light fixture down.


  Here's a better step-by-step than I can come up with: How to replace
  a light fixture.


Carefully un-thread the wires back down the chain to the broken loop and disassemble the chain/loop connection. Remove the broken loop from the top of the chandelier. 

Install the new loop, feed the wires through the center hole, open up the last link of the chain and bend it back together around the loop (This is a great time to make adjustments to the length of the chain if you want to) then feed the wires back up the chain, through the metal plate, and re-attach the wires and re-install the top plate, etc. 

Best of luck!