That's probably not going to recover.
Go find a new, pretty phal with good roots and start again!!
I have repotted phals from sphag to bark no problem, and I have repotted phals back into sphag no problem.
Perhaps this one was just sick and destined to die. I have had two oncidium types that arrived to me with a brown soft rot illness that killed them both in about 3 weeks very slowly
So you might have just picked up a sick phal and maybe the media change wasn't the cause.
Idk, we all have killed a phal or two. If you really want to try, just pot that up in some moss/bark mix and water it normally, and there is a chance it could put down some roots. It is really unlikely, but investing a few minutes to pot it and a few minutes a week to water it isn't that bad if you really feel desperate to try to save it.
Normally, you'd want to have at least one good healthy root to keep it going.