Why does my game not work after reopening unity?

After putting together a working build, I quit out of unity. I find a bug, however, and so reopen unity to fix it. Before changing anything, I try playing the game again. Nothing works. A few print() statements later, and I learn that start() runs on the main script, as does Update(), but FixedUpdate() only runs once. No input is registered.

So, concerned, I import a backup that had been saved an hour or so before the build. Without changing any code, I open it, and still something's wrong. I try a very simple scene, and notice that this time input works (maybe because the script checks for it in Update()), but none of the animations do. As in, even for a gameObject with nothing but a sprite renderer and animator. The animation controller has no transitions nor triggers; it exists only in a default state. The object enters that state, loads the first sprite, then stops, and the animation doesn't run. It's as if time just no longer works in the game. However, it's a "press any key to continue" kind of scene, and pressing a key does load the next level...

I try doing a build, but the screen is black and nothing works.

My eternal thanks (in advance) for any help with this problem. I'm very worried, a little devastated, and extremely confused. I get no error messages nor warnings, but let me know if there's any information that might shed light on the issue.