Do you think it froze back? Otherwise around here many of them appear to be going bald due to foliage mildew. If you have that also it seems you will either have to start spraying every year or give up on it.

The stems are too thick for loppers?

You could try cutting into one of the old stems to see if it is live.

Not mildew but removing old insect infested stems fixed my sickly Nandina, new vigorous growth hasn't been reinfested.

With the species nandina, new growth will come from the base (root crown) and it will always be more vigorous and robust than older canes. I would try to remove the oldest growth in the same manner that one does for deciduous caning shrub rejuvenation.

Need to identify cause of defoliation before knowing how best to proceed.

dead is dead.. remove dead parts ...

do it the best you can.. and if you injure a few new shoots.. so be it ... it will make more ...

keep in mind.. there is a HUGE root mass ... so dont be surprised how fat it might grow this year ...

a sawzall with a tree blade might come in handy.. if you have one laying around ...

ken