I would find a thick stem on the plant and scrape a little of the bark off of the stem and see if there is any green under it. If there is, then the plant still has life in it. If you find it still has some life left in it, I wouldn't do anything until it starts to warm up some more and you see new growth. Or, maybe selectively choose a few branches to lightly trim back and watch those for a week or two to see if you get any new growth on the stems you cut. If you get some growth on the cut stems then I would proceed to trim the rest of the hibiscus, lightly though, I wouldn't do a heavy prune as your hibiscus could be in a fragile state if it did suffer a lot this winter. Just my opinion, others might have more to say. Hope it works out for you. Let us know what happens!

Evan