what is wrong with my dendrobium and how to save it?

[lisaqiu]

Hello everyone, I’m new here and also new for Orchids.
I’ve got a Dendrobium Orchid 3 weeks ago. Not sure what exact type is this Dendrobium.
It got 3 flower spikes and all are well blooming.
But the bodys of the Dendrobium is shrinking and its loosing its leaves. The leaves turns yellow from the tip to the bottom. I am afraid I got the Dendrobium overwatered, the roots seems all got rotten, they were brown and full of water. I trimme them off (hopefully). And I have stopped watering for 1 week, but the roots are not getting better, and the leaves are one by one turning yellow.

Could you give me some advices about what I have done wrong with it, and what can I do to save this orchid.

Thanks a lot.