Yikes, Lily, you're right - that's one unhappy camper! Boy that's some difference from before the winter season. When I saw that mass of roots I thought they looked pretty dessicated. I think you've nailed the problem - rehydration.
The important thing to remember is that you can't fix the problem overnight. If you overwater it in you mercy, you'll drown it. You're looking for a long term fix, not a short term miracle.
My choice, knowing many here will have different preferences, would be a loose packed sphagnum (New Zealand if you can get it) for potting media. I'd work the moss in and around that root mass as much as possible for best coverage. (Soak the roots in luke warm water for 15 mins and try and tease apart as best you can. Looks like you'll need a larger pot while you're at it.)
Once repotted, I'd water just as the surface begins to dry out. If the moss isn't packed too tightly, that should give the roots plenty of air and moisture at the same time. If the plant is tippy, stake it rather than packing the potting mixture more densely. (That will kill the air spaces and promote rot.)