Thank you for getting back to me, Pat. Once a Japanese maple starts to leaf out in the spring, it is very susceptible to cold snaps. The new growth will freeze and usually turn black. However, a cold spell doesn't kill the large branches, just the new soft growth. when a branch snaps off, it is dead. That part of the tree will never grow back. That can be the result of an extra cold winter, too little or too much water, nutritional deficiencies, damage to the roots, or toxins. For example, if a root was cut off during a yard or construction project, at least part of the tree would die. Too much mulch or soil on top of roots that surface can also be harmful. It's best to leave the roots that come through the soil uncovered. However, the soil you added is also not likely to be the source of the problem.
All you can do now is prune off all the dead branches, and then give the tree excellent care. Prune back to living wood, and new growth will usually sprout from the cut. Fertilize with a high-quality fertilizer that provides macro and micro-nutrients.Osmocote is a good one. If you can't find it in a local nursery or garden center, here is an online source in the UK:http://www.lbsbuyersguide.co.uk/growing-media/slow-release-fertilizers/osmocote-exact-controlled-release-fertiliser.html
Throughout the growing season, be sure the tree is evenly watered to avoid too much moisture or inadequate moisture.
You can take the above steps on your own, but it would be better yet to consult an arborist. When a tree is under this much stress, pests and diseases can gain a foothold and an arborist cna help you prevent that. A good arborist will also know how to prune the tree to promote a nice shape, and how best to apply fertilizer. Here is where you can find a certified arborist:http://www.isa-arboriculture.org/
If you have more questions, let me know by clicking on REPLY. I hope that whatever approach you take, you'll be able to save your tree.