"It's the same thing that happened to me, except worse." Luna told them.
"What do you mean?" PinkiePie asked.
"I mean what made me turn into nightmare moon!" Luna said. The all looked at each other.
"Do ya mind tellin' us? Applejack asked.
"Okay. I became angered because my sister's powers were more loved than mine. So, I wanted to make an eternal night." Luna said.
"Then, what made Twilight go crazy? Who's more powerful than her?" Spike asked.
Luna took a deep breath. "Me." She said. They all gasped.
"Then, why couldn't you stop her?" Rarity asked. "Because, the kind of magic that I know iys far too powerful. If I were to use it on her, she would die." Luna said. "I can't have her remember all of the memories of you all- she would have to stay still."
"I'll go to the library, there has to be something under that rubble." Rainbow Dash said. "But, your wing!" Rarity said. "Well, it's been a few days with it like this. I have been able to move it around pretty well. I should be able to fly just fine." Rainbow Dash said. They all looked around at each other. "Go ahead." Fluttershy said. And Rainbow flew out.
"Do you think she'll be okay?" PinkiePie asked. "I'm not sure. But, she wanted to help. I think that if she believes in herself strong enough, she can." Fluttershy said.
"I sure hope you're right." Applejack said.
Before I even touch this with a reviewing stick, it needs to be properly formatted. A dozen lines of conversations are in the same paragraph in the second chapter; several paragraphs are stuck together, making it displeasing to the eye and difficult to read; and you need to get the punctuation straight.
I direct you to the Writing Guide.
Outside of the fuzed paragraphs and hurried progression, your story chapters are too short, really anything less than five hundred words could be considered a skeleton draft. If you want to add meat to your story, try fleshing out each scene with details; don't just tell us what happened, actually show us with imagery.
Another thing to watch out with, all of the ponies in this story seem to be taking these events extremely well. I would have imagined Fluttershy to be freaking out if something like this ever happened, yet she speak perfectly fine, only slightly sad as she stares into the dying stallion's eyes. Even Rarity and Applejack seem unfazed by the deaths of Apple Bloom and Sweetie Belle.
Since it's your first fic, it isn't going to be the best thing in the world, but don't let that discourage you. Take in any criticism you get and use it to better your writing skills as you progress in this fandom.