A photo would really help. Do you mean a Japanese Acer or one of the much larger ones? What situation is it in i.e. sun, dapple shade, shade? Is your soil acid or alkaline/ Is it in a pot or in the ground? Sorry but really can't help without more info.

If its died from the top down, its got dieback, which often occurs on Japanese Acers - there is no cure I'm afraid. It sometimes means trouble at the root, but there are various causes. All you can do for now is cut it back to remove the dead parts, but you'll probably find its on its way out, unfortunately.

If its not from the top down, then a photo would be very useful.