Search these forums for "restoration pruning". I posted a link a couple months back after the ice storm. Without seeing it, can't sa for certain, but it probable can be recovered, but it take time and careful pruning. What cultivar of JM is it? Makes a difference.

Arktrees

Is it severed above or below the graft union? If below, all is lost and the potential you have is a nice tree from the rootstock. If it broke above the graft union, you can certainly hope to rebuild your tree in a form that pleases you, with a little patience and judicious pruning.

I would let it grow w/o pruning anything for a year - maybe two or even longer, depending on how the tree progresses. Let the tree put on some branching and tell YOU what it wants to be. You'll be happier with the results if you just 'go with the flow' instead of trying to buck the natural growth habit.

Al

weeping maple split middle of trunk, branches on both sides one already fell off with a little help. Frost results. Should the split be covered and with what?