Try rebooting your PC. Sometimes that fixes the game not starting.

Sometimes installing pending windows updates fixes it.

I don't think anyone knows why.

This game is 19 years old (2006)
Briefly, its install folder handling predates both Steam and Windows.
It doesn't play nice with them, and vice versa.

Hence Windows might introduce new conflicts beyond SE:V's 2006 scope:

• if your Steam root is your C:/
  
• if you use Windows OneDrive, or any other path-munging service
  
• if app uses a since-deprecated API to open folder (c.f. Stardock)
  
• other

The Windows Update tip makes some sense.
If a (newer) Windows service quietly munges some filepaths to give SE:V the illusion
that it owns its own folder, but an Update disables that service, that can break paths until.

SE:V might be the oldest app you run on your PC this year. (Go launch SE:IV )

I find whenever the game mysteriously stops loading it's time for a computer reboot which has always resolves the issue. If I had to hazard a guess to the cause it's probably some sort of steam update replacing a system file that needs a windows restart to load.

Did you change your save file compression from true to false or the other way?
That kills saves from prior to the change.