Get tough and cut then to their ankles - or maybe their knees. Cutting back all the damaged growth so that the main stem is only a foot or so tall should shock the plant into sending up lots of fresh new growth - or the severe pruning will kill the nandina for good. Either way you won't have ugly shrubs  anymore and in a month you will either be delighted with the new growth or be heading to the nursery to replace the plants. Nandina are tough shrubs. I am going to go out on a winter damaged limb here and predict your Nandina will be growing fast and looking great in just a few weeks. Add some slow release plant food like Osmocote around the roots once you see new growth.  Keep growing, Marianne Binetti