With a lot of time on your hands you will have to sit and untangle by hand. If it is that bad, it may be easier to buy a new one.
Connie.....If it's a thin string you might try and needle or even knitting needle
It is the same technique you use to untangle your jewelry chains, etc. It takes a lot of time and patience. Go to the know and put something like a needle or chopstick into the center and start to wiggle it until it loosens and just keep working at until it is undone.
Sometimes just rolling it between your thumb and forefinger will loosen the knot. It might take a couple of minutes, but I’ve saved both string and necklaces this way.