Lay it out flat on a piece of paper, and have good lighting. Use a straight pin or something else very very small to pick at it. Don't just pull, try to actually undo the knot. You may also be able to use something like cornstarch to help lubricate the chain so it doesn't get as caught up.
If the chain is closed, like the clasp is attached, leave it. It was tangled up like that so you should be able to untangle it. Keyword is should. If you can't get it, undo it and work with just one strand and loop and unloop it from the knot until you can't get any further, then switch to the other end.