It is ok for orchids to lose their flower spikes after they are done flowering. However, when all the leaves start falling off, then something's wrong. How are you growing the plant (like watering, temperature, light, etc)? Without your growing conditions, I cannot tell for sure, but I can make a guess and say that your orchid has root rot. Without roots, the plant cannot take in water, and thus becomes dehydrated (shriveled). If this goes too far, then the leaves start falling off. Can you check the roots? If they are firm, that's good, if they are mushy and hollow, then that's root rot.

Here is a link that might be useful: AOS oncidium culture sheet