I'm sorry to say I agree with Philip. I try and save mine and don't give up for a long time, but I would be throwing that one
They tend to either need healthy leaves (I think those remaining two are almost dead and could drop off soon) or healthy roots from which new leaves can grow, but the one bit of green root is unlikely to be enough.
Sorry to be so down, but if I were you I would console yourself by buying
two replacements
If I had to guess, I would say that the roots of this rotted, then the leaves were not getting enough moisture and died as a result. Growing in moss can be difficult (though it's a matter of taste as some people do better with it than bark).
Mericloning is not really something you can do at home, it's done in a lab and needs a lot of expertise. It takes some cells from the plant and reproduces them. If you are not familiar with the process and don't know someone with the expertise then it's not really a viable option.
If I were trying to save an orchid, I would do as Abbie says and cut off all the dead root. Then pot it into a tiny tiny pot (an old yogurt pot or something very small, it should be only just big enough to contain the remaining root). These days I would pot it in lecca/hydroton which is clay beads, but in the past I've done this with bark. Cut slits in the sides of the pot, and rest it inside a bigger pot just to keep it upright. I then try to get it drying daily and I water daily. You should only water when it dries, but rescuing them works best when you can water very regularly with quick drying. I also water rescues with luke warm water and use seaweed extract to try and prompt root growth.
However having said that I really do think you could be wasting your time with this one. Really sorry