It sounds like wind damage - I have an Acer palmatum and the leaves have (mostly) dropped off after a severe wind.
They should grow back in the spring, but in the meantime, see if it's in too exposed a position.

It's not a new one, is it, that hasn't been watered enough? Just another thought.

i think as a general rule acers dont look there best for the last quarter of the summer . mine are not looking there best now but there fine . if you took a photo when the leaves have just opend and one now with the sun in the same place i think youd be quit amazed how different in colour etc they actualy are .