What do I do when the clivias die back? Do I cut the stem back hard?
Clivias are not annuals and they do not have a true dormant period. They need protection from freezing temps and they should be allowed to get drier than normal during the winter months, but not enough that the leaves dies back or even will.
Apparently, yours did get unusually dry. It is hard to know how much damage was done to the roots. Remove all of the dead leaves, but do not cut the main stem or repot the Clivia. Do saturate the rootball initially and keep it close to a north window. Be patient and look for signs of healthy new growth that will signal that it is recovering. Good luck with it!