I had read a story a long time ago when I didn't have my account but it had twilight locked inside her castle away from all the windows and no one has seen hide nor hair of her till Luna decided to check up on her, more or less Luna found twilight in a very bad state and near death due to a strange illness that grew in strength when she was ether in sunlight or in Celestia's presence.
I think it had something to do with a ursa minor twilight put back to sleep, not completely sure, and I don't know if it is complete or not. Also this is based after tirek's defeat and before the cutie map I think.

It's called "Almost Too Late" and it's one of my favorites. Here is the link to Chapter 1.

https://www.fimfiction.net/story/215302/2/all-for-one---one-for-luna/01-almost-too-late

Enjoy

Oops. The title is "All for One- One for Luna" . . . Almost Too Late is the title of Chapter one.

6601277
Thank you so much, also I'll leave this here if anyone else want to find the story.

6601345
You're welcome.