Hi, cryptojoe.

Could be a simple hidden vertices issue.Try this: Select the object, enter on de Model Room and go to View > Reveal Hidden Vertices. Or, sometimes Carrara's shows a message at the top of the window "Rendering Failed". On the Model Room, turn off the Subdivison Smooth, sometimes this may help bring the model back.  And don't forget to install the 6.0.3 patch too, is very important.

Cheers!