I have used the product viewer to draw the top of my cabinet in 2D.  I added some holes and then updated the part back to the product.  I've done this or something similar 1000 times, and of those 1000 times, I have had this problem with two products that I was drawing.  The problem is that when I select Update Current Part and Redraw Product, I get an unhandled exception error.  When I clear the error, all of the parts in the carcass subassembly have disappeared.  The parts in all the other subassemblies are fine, but the carcass is gone.  If I ask for the parts list for the carcass, it is empty.  If I go to the design data and show the cut parts for the carcass, there are none there.  Close Microvellum without saving and reopen, no carcass parts.  I don't know how to get them back without starting over on a project that I've spent 2 days drawing.  There must be a way to get the parts back.