Japanese maples are prone to a number of die-back type diseases, some more serious than others. Several are very weather related and even those that are not tied directly to weather may be indirectly tied - out of season or extreme weather conditons can stress the trees which makes them more vulnerable to these pathogens. So last year's drought, hurricane-type winds and heavy rains, late freezes, etc. could all create a stressful environment and increased sensitivty.

Pruning out dead wood is always appropriate and may be helpful as well. Be sure to clean/sterilize pruners well. Keep adequately watered this summer and as necessary later. By next spring you should know whether this tree is worth keeping.