Using your thumbnail, scratch at the bark near the top of your Japanese maple to see whether the tissue underneath is green (living) or brown (dead). If it is green, theres still hope that your tree will recover and releaf. If its brown, you can safely prune off the dead parts. Even if the whole top is dead, the plant can start over from suckers. The root system that once supported the whole tree can now direct all of its energy into the suckers, so regrowth will be rapid. An application of a granular slow-release balanced fertilizer (10-10-10 or 12-12-12, for example) will assure that nutrients will be readily available when your maple needs them.