Answer: The top branches could be bare due to winter cold/wind damage to the branches, or due to spring frost damage to the buds. If it is just spring frost damage due to oscillating temperatures, and the branches are still alive, then they should leaf out.
I am a little worried it may be winterkill. Most Japanese maples are hardy into USDA winter hardiness zone 5 at best (many are only hardy to zone 6) and your zip code places you in 5A or the coldest part of zone 5. In a windy or exposed location the microclimate where it is planted could be as cold as zone 4. This would mean it may be prone to winter damage due to not being able to tolerate your typically cold winter weather. If there was winterkill or dieback to the branches, then you would need to trim off the dead wood.
You can check for live wood beginning at the branch tips. Live wood is supple or flexible and has green inside the bark. The bark on dead wood is dull gray or brown and it is brittle, it will snap in your hand. Trim off any dead wood as it will not regrow.
The weeping form of Japanese maple is special in that the branches should be allowed to reach the ground. This is the tree's natural way of shading the soil to help keep it evenly moist.
Place a layer of mulch about two inches thick under the tree and all the way out to the branch tips and even a foot or so wider than that. (It should be no deeper than two inches, spread it out in a flat layer and do not allow it to touch the bark or trunk of the tree.) This will help keep the soil moist and will also help feed the soil slowly as it breaks down over time. It also makes it so that you do not have to try to trim or mow under the long branches.
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