Cold damage to big Phal

[Groq]

Hi there, I'm brand new to this forum, but reaching out for a little advice. I have a 5 year old Phal that looks like it isn't going to pull through a really cold new year. We had a gas leak at our home and had to shut the heat off while repairs were made over the new year weekend. Temp in the room got down Possibly below 40 F. The plant is big and went totally limp and leaves are getting dark. A smaller Phal I've been rescuing pulled through no prob and is showing new growth already.

The roots on the big one seem to still be plump in places. Is there a way to save the root system and cut out the dead or dying? I may try if there is no other hope, but would love to hear your ideas. Also, can someone explain what happens to the plant when it gets so cold and whu a small one does well but a big one bites the dust.