I'll be working on a figure, rendering every once in a while to check it, then without any reason, the figure will not render.  All I get is the background - no figure - and no matter what I do I can't get it to render again.  I've saved and shut down.  Restarted the program.  Restarted my computer.  What the heck is going on?

 

This has happened with several different figures in the last two days.

Any help will be appreciated.