Everyone who has ever owned a fine jewelry chain has dealt with this one, and getting it sorted is a bitch (which is the reason the bottom of so many jewelry boxes are covered with hoplessly tangled necklaces). With luck, perseverance, and endless patience you can sometimes tease the knot out, but there are actually easier ways to tackle it.
1. Put a couple drops of cooking oil on it, then lay it on a flat surface and use two sewing pins to unpick the knot.
2. Put the chain in a baggie with some baby powder or cornstarch and shake it. The powder will loosen the knot and make it easier to untangle.
I’ve tried both of these methods and they actually work. Getting the knot out is still a time-consuming pain in the ass, but if you really do want to save the chain it’s worth the effort. The cooking oil or baby powder/cornstarch really do help enormously.