A flash freeze can cause leaves to turn brown and drop off without going through the usual phases of dormancy in the fall. I would mulch them well for winter protection and see what happens in late spring. It could be the roots are fine. But, you will need to patient in the spring watching for leaf buds. It might mid-to-late May before you see signs for life. Once the crapemyrtles are established they better handle extreme temperature swings. Though they are more like trees in the lower elevations of your area, they might be more like shrubs in your area.
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