Greyscreen when rendering

I started my render of a scene at 2:00 am.
I went to sleep and let it render. Now it’s 10:30, and Blender has gone completely grey (like completely the whole screen is grey) and has produced 4 pictures. My PC is still running and shows activity (200% CPU usage??, 1GB of RAM being used. I am not sure what to do at this point, as I do not want to stop the render (if it is still rendering), but the grey screen really has me going for a restart. Why could this happen?

As I was writing this a 5th Image got rendered.

What do you think I should do now?
With this render speed this will take a week or so… I am rendering the standard 250 Frames at 4k resolution and 30fps and I am using Cycles. The scene contains a fluid simulation with a resolution of 160 for renders. Took about 3h to generate.

Ok so the basic Issue is solved, the greyscreen is gone.
I moved the window from one screen to the other and it is working again.
But how could I speed up render time??