Answer: Heavy snow can be so damaging, it is important to try to periodically sweep it off of fragile plants before it builds up enough to break them, but this is not always possible, especially if it snows in the night or when nobody is home.
Sadly, there is usually little that can be done when a branch is broken off, but you can try. If you can align the pieces together perfectly (like mending broken china) so that the inner circulatory system is matched back together, there is a chance it could heal just as an intentional graft knits itself together.
You would need to match up the broken ends, then hold them securely together and support the weight of the wood. It will probably require a splint and perhaps some tape to support the pieces. The ideal "glue" sealing the bark outside of the joint (to keep moisture and insects out of it) would be grafting wax, usually sold at garden centers. Finally, the procedure would ideally be done immediately when the break is fresh rather than some days after the accident.
On the brighter side, a young tree will grow relatively quickly, and a tree with an established root system will grow back faster than it grew originally. So the tree should regain its size fairly quickly even if your repair is unsuccessful.
If you decide not to try to repair it, then neatly trim the broken end, cutting just beyond a small branch that is already heading in the direction the new growth should go. The small branch will then become the replacement for the one that broke away.
I'm sorry about your tree, it is always sad when this happens.
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