Smooth proxy disappearing.
 

I working on the Joan of Arc tutorial (Maya version), but now when I try moving the vertices on the poly model, both smoothed proxy's disappear. (The original and the duplicated)

Re-opening the file gets them back, but as soon as I move a vertice, they disappear again.

Any ideas?

Try deleting the history and if that doesn't work delete the proxy and make a new one. And remember to save copies of your work so that you have something to fall back on.