I doubt they have died. Heat is more an enemy to japanese maples than cold. I am certain they will regrow their leaves.

They'll probably be fine, they have the ability to put out another set of leaves. With that said, if it's going to get down that low again, bring them inside.

This year will be tough on a lot of trees, as 1.) We had a record warm March, so everything is further along than it should be right now, and 2.) They are forecasting record lows for many areas this coming Saturday. I know some of my Japanese Maples' leaves are wilted and black because of a few cold nights already. I plan on covering them with a frost blanket this weekend to try and prevent any additional damage. We'll see if it helps.

Japanese maples are capable of a couple of flushes of foliage although it is not necessarily to their advantage to have to experience multiple flushes in a single season. And late frosts can be very damaging to JM's, especially those in containers. I recall a couple of years ago when much of the eastern half of the country experienced an Eastertime hard freeze and the reports of stem dieback or even tree death as a result were numerous. Light frost are tolerable once the trees have emerged from dormancy - hard freezes not so good.