Question
I've had a phalaenopsis orchid for about four months or so. Everything was fine or at least I thought so but since two or three weeks ago my orchid started to lose its roots I had to cut most of them since they were mushy and hollow. I did the sphag-n-bag thing and it seemed alright but when I checked on it turns out that the stem had started to turn black and I think it has fungus on it. And since it has no roots and all that I was wondering if it could be saved. The only thing that seems healthy is one lonely leave that's still strongly attached.

Answer
Alejandra, apparently, no one had made you aware that your plant should have been repotted as soon as it went out of flower.  The pot should have had lots of drainage holes and the plant should have been repotted into a fir bark type of orchid potting mix.  Old potting mixes tend to stay wet too long and lead to root rot. I think at this stage, chances for survival remain slim.  Since it has a healthy leaf, it can grow new roots.  You will need to repot it in a small pot and keep it on the drier side.  If you bag it in its pot, use only a strand of damp sphagnum and no other moss in the bag.  The only purpose of the sphagnum is to provide a source of humidity in the air so the plant doesn't dry too much.