I'm sure it could have been many different things. She may have broken her neck, hitting something as she flew off the roost, maybe a wall, a nest box, a feeder or waterer. I found one that I think hit the feeder on the way down. She was so limp, I could not tell if the neck was broken or not.
The way some hens panic trying to get away from some roosters, it would not surprise me for one to hurt herself by running into something or getting her head caught while trying to get away. At seven months she may not have been mature enough to accept the rooster, although I've seen hens of all ages resist some roosters.
Possibly fatty liver syndrome. That's basically obesity. If a chicken gets so fat that globs of fat build up on the liver, gizzard, heart, and in a great glob at the vent area, it can kill them.
Some just have genetic weaknesses. At some point, something just quits working and they die. Nothing you can do about that.
If she ate a lot of calcium too young, her kidneys may have been damaged. They die and you don't know why.
It is possible she got into something poisonous the day before.
The other things I can think of would have probably been more of a lingering death, not quick, though I'm sure I'm forgetting some things. I've had three die suddenly like that. I have an idea what may have happened to two of them but with one, I have no idea. When you keep living animals, you sometimes have dead animals. I get a lot of pleasure out of mine, but sometimes things happen that are not pleasant. I'm sorry you are experiencing that.
