Hi, I have had the same with a A rutenbergianum and an Aerangis biloba. Sudden leaf drop (with leaves a little brown at the base), approx 1 leaf dropped per day, good root system with no damaged roots, no obvious fungus or bacterial infection. I never really found out what it was. I would say that a cultivation issue is unlikely: you may loose one or two leaves and the plant may generally be stressed.

My suspicion is that at least in my case this was fusarium wilt (blocking the vessels distributing water around the plant).

Can you post a picture of the plant? If it has no leaves, it's unlikely that it will come back. As it is possible that we are looking at an infection isolate it at least. If it is something like fusarium, you would see some purple colouration if you cut through the rhizome.

Help! My Angraecum didieri dropped all its leaves!
 

UPDATE:::

Hard as I tried, I could not save this plant. The main stem turned black . . . . there really was no way this could have been salvaged...

It has gone to "Orchid Heaven" . . .

Thanks to all of you for your assistance and your concern ..........

Clearly a fungus infection then

Sorry to hear that :( Let's hope, if it was fungus, that nothing else goes the same way