If it is a Steam cloud save you won't see the file there. They are accessible here instead:
store.steampowered.com
The most recent game patch is 1.7.5 over a month ago. If you've played it since then, I can only guess it is indeed corrupted in some way, OR you accidentally loaded it with a mod or mods enabled.
Such corruption can be caused by game bugs, or issues outside the game. It may or may not be rescuable depending on how severe it is! Your best resource to look into that is probably the good folks over in User Mods who are more used to editing and analysing save game files.
Sorry for that problem, whatever the cause!