Hi, I used to have similar issues on my old system - it turned out to be Windows going into power save mode two hours after I'd left it alone.

What I ended up doing was going into my control panel and turning off all the functions like 'sleep', 'power save mode' and then in the system management tab, turning up automatic updates as this can prompt the machine to restart.

I then manually switched the screen off, put the keyboard upright so the cats couldn't land on it and went to bed :) all worked fine. 

The next day I just re-enabled the power save modes and automatic updates. It only took a couple of minutes, once I'd figured it out.

Best of luck! I know how frustrating it can be to see a blank screen or half a file where you're hoping for a beautiful render.