Phal with very dead roots

[pbridges]

I'm new to orchids, and a friend gave me a Phal for my birthday. All was well until about a month ago, it had grown new leaves, and I put it in a east facing window.
I guess the room was too cold, cause the bottom leaves turned yellow and fell off.

So, I moved it somewhere a little warmer, and it still lost leaves. Finally repotted it yesterday - most of the roots were dead. I cut them back, applied cinnamon, replaced the spaghnum moss with bark (the only orchid soil I could find locally) and now I'm concerned.

How many roots can you cut off before the entire plant dies? I've put it in a warmish room (69-70 degrees) and plan to leave it alone to see how it does.