Render files... gone

I had rendered a job for 2 hours, and everything's been fine for a few days until 10 minutes ago.
I QCd my spots an hour earlier, closed the project and just reopened it a little later, to discover that half the rendering is gone.
Maybe I should specify that I had a new gfx endpage that over wrote the original one, so the spots neeed re-rendered on the endpage portion only.
SO 50% of the spot just need render on this endpage portion, but he other half needs full render.
DOes this make sense to anyone?
Thx