I do have two mods installed via Nexus Mod Manager: Fallout New Vegas Anti Crash, and the one that adds more fast travel points, most notably the Vegas Strip area. I looked to see if any driver updates had auto-installed overnight, but there was nothing. It's not just one save file, I tried several different ones and none of them are playable. It worked one day, and the next day, it didn't work.
I'm totally stumped as to why my game suddenly is unable to load any saved file.