In all liklihood, you'll simply lose some top growth. Don't cut it back until you can be certain where the stems are actually dead. It may (or may not) be that simply the leaves were killed, but not the stems.

You'll find out when it's time for the new growth to appear.

I had 4 small citrus trees last year that did not protected from the cold except for a temporary wall that I constructed around the trees. The tops of all the trees turned brown and they continued to take a down turn even during the next 9 months. One of the trees finally died but the others came back. I have one large tangelo tree that was 6 foot tall in 2000 and it was the only survivor of 6 trees that I planted in 1998. Five out of six trees died from the 20 degree weather but the tangelo tree froze all the way down to the graft. It came back slowly at first for 2 years and then it took off growing like a rocket. Unfortuneately, we just had another 2 days of 17 degree weather. The trunk of the tangelo tree is about eight inches in diameter. All the leaves dropped and I am waiting to see the end result. The tree was too big to feasily cover so I just hope that it can come back and be climatized to the severe cold.

Wait until spring and see how far back that you need to trim the brown limbs. Cut them back and apply a pruning sealer to the ends to prevent an infection to the limbs.

I agree, don't prune anything at all until you have significant new growth so you can be certain what the tree can preserve. It knows best, let it rest.
Arthur

Did your heater go out? How did it get so cold in the greenhouse?