These plants are happiest in zones 7-10. If you are near Berwick then you are probably in zone 6 so borderline, Try the Lagerstroemia fauriei which is a bit hardier, perhaps 'Sarah's Favorite' which is white or 'Velma's Royal Delight'which is magenta. Of course this doesn't help the plant you already had. I guess it got clobbered by a very severe winter and it will recover. This shrub will sprout from the base even if the top growth has been killed. The roots should survive as far north as Massachusetts. You should definately prune any dead branches, check by scraping the bark to see if it is really dead before pruning the branch. You could try protecting the plant in winter of course - a wire cage packed with straw over the entire plant will protect it. Of course this can't be done practically where it has grown very large.

You can grow this plant by pruning almost to the ground each winter (a practise known as coppicing). Of course this means that the plant stays very small but this can't really be avoided and as it flowers on new growth then you will get a lot of flowers each year. This is my recomendation - pune the dead growth this year. If the plant gets clobbered again next winter go the coppicing route and buy some more hardy varieties.

In any case general care: Do not feed at all or water too much in Autumn. Mulch in spring with an organic mulch. Mulch deeply (at least 3 inches)with wood chips, pine needles, chopped leaves or pine needles. This will help to protect the roots in winter. Feed in spring with an organic general purpose fertilizer.