Game crashes constantly but didn't before

Right click on ck3.exe , properties, compatibility. Tick "disable fullScreen optimisations" untick every other box in that dialog, Apply and exit.
Then in the launcher Game Settings menu, select fullScreen display mode, Vsync off, cap refresh rate at 60.


If those don't help:
DXDIAG is a program you run from the Windows search box on the task bar. After running it will open a window and start collecting info with a progress bar in the lower-left corner. When it completes click the 'save all information' button and save it to a file then attach that file here.

Please attach here your Documents/Paradox Interactive/CK3/pdx_settings.txt .
From your Documents/Paradox Interactive/CK3/logs/ folder, attach here system.log , error.log , exceptions.log  

Yes, I'm afraid this machine does not have enough system memory to run the game, and the CPU AND video system are below the requirements:

Processor: Intel® Core™ i3-2120 / AMD® FX 6350
Memory: 6 GB RAM
Graphics: Nvidia® GeForce™ GTX 660 (2GB) / AMD® Radeon™ HD 7870 (2GB) / Intel® Iris Pro™ 580 / Intel® Iris® Plus G7 / AMD® Radeon™ Vega 11

You were lucky it ran at all before, but it simply will not now. You could revert to a game version from way back then, in the Steam Betas tab for this game.

Failing that, I hope you have access to a machine that meets the minimums.

Sorry for the bad news!