Treat her for shock if needed - keep her in a dark, warm, quiet place. Make sure she is eating and drinking.
You might want to get her another friend sooner rather than later.
So sorry this happened to you and your hen :hugs
Chickens can't see in the dark at all, so they freak out when the lights suddenly turn off. Mine always settle down after 10 minutes. You can try to place them on the roost in the evening until they get the hang of it.
A few ways I've heard are
- plenty of cool drinking water (obviously)
- providing a shallow tray of water for them to stand it
- cold or frozen fruit treats like watermelon