My game shuts itself down

@BLsWnd  Does your computer run Windows or macOS?  Do you have a laptop or desktop?

And why are you leaving the game open while away?  If Sims 4 runs fine except when it's open for hours on end without your being there, then the easy fix would be to quit before leaving.  It's entirely possible that your operating system is trying to update and detects that you're inactive, then triggers the shutdown/restart and closes your game in the process.

It runs windows, it's a laptop.

@BLsWnd  Please run a dxdiag and attach it to a post.

https://help.ea.com/en-us/help/pc/how-to-gather-dxdiag-information/

I will reiterate though that Windows is happy to install an update and restart or shut down your computer if it detects you're not actively using it.