At this point in time all you can do is wait and see what happens next. If by June 1, the stems show no signs of growth, prune them back half way. Then scrape the cut off pieces with a knife and see if there is any green. If yes, there should be growth coming from the stems left on the bush in short time. If by July 1, no growth comes from the remaining stems, the new growth emerging from the bush's crown will restore the bush in due time.

Easy on the fertilizing of the recovering bush. It doesn't need as much as the bush next to it until it catches up...may n be until next year, so be patient.

Moses