This is a very important information about a problem I had recently where all objects in the Blender file disappeared when doing some tests in Blender 2.79. I’m not sure yet where the problem is, but you could lose all your work, be careful and take regular backups of your files.
I don’t know if this is a bug in Blender 2.79 or in the interface between Blender and YafaRay. If you have experienced this problem when you were using Blender Internal or Cycles (and not YafaRay) please let me know.
More info here: