My Dyakia hendersoniana is dead.
 

That's a shame. It's sounds like you were doing everything right. Have you checked the entire length of the stem for hairline cracks or breaks? Sometimes an old injury can be present without our knowledge. This happened with a vanda of mine that was given to me last Christmas. It had a sizeable keiki and was in bloom when I received it. The baby plant looked fine, but momma's leaves were a little dehydrated and her existing roots were dry and cracked. I figured this condition was an easy fix and bumped up misting sessions to encourage fresh root growth. No positive changes took place in spite of my 3 month efforts. When her leaves began yellowing and dropping at an alarming rate I discovered she had a break near the tip where leaves emerge. The strength of her leaves had kept her growth erect and the damage hidden. It was all downhill from there, but I did manage to save the keiki.

Sometimes they just decide they dont want to live anymore. Suicidal. i had couple of those.

sorry to hear this...if it makes you feel better a orchid grower once told me,if you kill 100 your a good grower if you kill 1000 your a excellent grower(something about learning from your experiences)lol,if you decide to try this one again,good luck :)

I wonder if they just get tired, had oldies run down hill and die no changes to conditions at all ... Sorry Tindo know it is flustrating ..

I have had similar problems with plants that I purchased (typically from overseas growers). If you don't get them properly started and growing within the first season, they either die in the winter or the second season. This is just happening to an Arachnanthe clarkei cutting that I got last year. It had good roots, but never took off. Only minimal growth no new roots. It lost almost all leaves in the last 3 weeks and is not likely to survive. Whereas other plants that I got from Malaysia at the end of June have started to get lots of new roots & new growth. Same conditions. Sometimes there is just an element of luck involved.

If the plant was doing well before, it could have been an infection through an injury

Species nut is exactly right. Kill 100 plants, you're a good grower - kill 1000 and you're a great grower. Rock gardeners talk about killing plants more than orchid growers do. I doubt that they kill any more plants than we do - they just talk about it more...
also - don't throw it away yet. See if it shoots up from an axillary bud. If you've had it a while, it may have enough power stored up in the roots to push up a decent growth.

Dyakia hendersoniana has never been an easy one for me to grow either. You're not alone.

I think this one grows in humid environments. They may not like growing in moss.

I gotta look up and see if there are any pics of these in situ...