Check the trunk from the broken part downwards - if its completely healthy, with no cracks, splits or soggy areas in it, then cleanly saw off the broken part, at an angle to allow rain to run off. It will shoot from that area, and probably from the base as well - if you prefer, you can just saw it down to the ground, but it will put out several new clumps of growth from ground level.

Sadly, they don't heal, but they will re-sprout, as Bamboo says. Gardeners should keep in mind, though, that even woody trees don't really heal if propped up, and will always have a weak spot at the split. Better to cut them down, and let them re-grow from sprouts, or replace them.