Can't help you fix it but a little piece of my heart broke when I saw the pic.

That is such a large piece of a very ornamental tree, I would probably start over again :/

Unfortunately that is a fatal break. At best you have half of a tree. I agree with jond03, time to go tree shopping

One more thought. When the wound was very fresh, you could have (and can still try) to literally bolt the tree together. If the wound is not fresh it will not work and may not work on this species at all anyway. Worth a try if you are going to toss it anyway. Just check the broken limb for health.

If it recovers, it will most likely not have as appealing a trunk until it grows for a few years.

Line the broken limb up where it broke, drill through tree in 3-4 places - top, middle(1 or 2 times) and bottom of tear, insert bolt and tighten nut until fit is very tight, wait and see.

I think a lesson is to be learned here. A Japanese Maple should be pruned to handle a snow load in areas that receive snow. Most people do very little, or no pruning, and the tree ends up looking like a shapeless mound that catches a lot of snow or ice.
I prune the weeping forms starting at the bottom and then working up and out, ending up in a thin umbrella that is not overly wide. I prune any time of year and I prune several times a year and I'm not shy about it. The trunk should be seen, not covered up. Beware of too much sun on the south side of the trunk and on top. They don't have to be symmetrical. Japanese Maples are a lot tougher than most people realize.
In addition, you can also plant the maple under another tree. In that case, summer irrigation would be required. Here near Seattle in our dry summers it's required in most cases anyway for maximum growth.
Here's one of mine.
{{gwi:1008918}}
Mike

Thanks for the suggestions everyone. Great picture Mike! You have inspired me.

The branch broke off on its own and I am going to let it go for a while to see if it can regain some of its form. I do think Mike is right - it is time for some hard pruning.

Thanks again.

Crown , In a situation like that i would cut a-b and clean up with a sharp knife. Then take and stake up that green branch to act as the new leader. It takes a while, but you may be surprised!

I think you should prune until the next possible leader. It might take a few years, but I agree given time your tree should pull through just fine and the old wound will eventually heal if pruned properly.

Perhaps the term is being used differently, but weeping Japanese maples have no "leader", therefore there is no reason to stake anything to create a new "leader". Japanese maples, even upright forms, belong to a classification of the growth habit of trees that is described as 'decurrent' or possessing weak apical control. These types of trees, which include most deciduous shade trees, grow a number of more or less equally sized major scaffolding branches (no single 'leaders') that will form a rounded canopy/crown - or in the case of weeping trees, a flattened, spreading crown.

That is a pretty severe wound. You could have bolted the split had the damage been assessed earlier (not necessarily an action I would recommend on a rather delicate little tree like this) but at this late date, pruning off the broken branch cleanly (A to B) is really the only viable solution. The tree may or may not generate replacement growth from this side but will likely always look a bit lopsided.

If it were mine, I'd replace it. And as botann suggests, prune or thin as necessary to allow for potential snow/ice loads going forward.