Japanese Maple Tree question
We could not tell much from your photos. Japanese maples are subject to many environmental stressors such as drought, poor drainage, too much moisture, temperature extremes, too much mulch, planting too deeply, soil compaction, etc. We have had excessive rainfall and hot temperatures this season and this can affect the root system. Look at the attached publication for more information on these types of problems. http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG86%20Common%20Abio...
Also, in cold winters and fluctuating temperatures in the spring sometimes the bark can freeze and kill the cambium which can cause dieback. If the tree is grafted sometimes the graft will die. If your maple is located in a southern exposure or a windy spot, it can suffer extremes from wind and cold. These trees grow best in a drained soil in morning sun and afternoon shade protected from winter winds.
Scratch the branches with your fingernail and look for green tissue. If you see it, the branches may still be viable. Otherwise, they are dead.
If viable, prune dead wood, make sure mulch is no thicker than several inches and keep away from the base of the trunk.