Why had this happemed?

Please see attached.

One moment all was good. Then suddenly, having completed a 'strip" layer, the existing layer went black obscuring the drawing within the surface outline (see cature1)

Where I have worked on cut and fill areas those areas then show up, But around that area you can see nothing of the baseline drawing (see capture2)


I've tried rebooting my PC - made no difference. I'd like to know what has caused this. What I can do to prevent it happening again and how to correct it.

Thanks