I stopped playing for a few days, and now the game is crashing on start-up.

Sigh.

There must have been an update, I guess.

The last update was 1.01 on August 12th. If it crashes on start-up, the game likely has trouble loading your most recent save. In fact, this could happen to any save. Any save may be unable to load no matter how many times you try. But I seem to have found a workaround. When that happens, note the location of the save that can't load. If it was saved at Location A, then load another save, one that was saved at a nearby Location B. If that, too, doesn't load, load another save, until one loads successfully. Once you are in the game, either travel on foot or fast-travel to Location A. If you succeed, load the very first save again, the one that takes you to Location A. And you'll see that it suddenly is able to load. Restart the game, load that same save again, and it is likely to load again, even though just a few minutes ago you weren't able to load that very save.

I suspect this phenomenon is due to the engine crashing while loading the assets into the game. When you load a save game, or when you travel to a different location, the game has to load the assets into the world. Sometimes one method works but the other doesn't. I've seen the reverse happen too: loading a saved game is fine, but running or fast-traveling to another location crashes the game.