I have two hibiscus (1 1/2 - 2 inch diameter and about 5 1/2 feet tall). They have been pruned back many times and the blooms did not start until about two feet off the ground. We had several days below 32 degrees. I covered the plants and base, but all leaves are gone with no sign of buds, even pruned all branches back at least a foot.
Is My Hibiscus Dead?
Certified GKH Gardening Expert
If the stems are still alive, they will regrow their leaves and there is also a chance the roots are alive and they will regrow from the roots. I would give them a month, and if you do not see growth by then, they are most likely dead. These articles will help you as well: