The flight to the castle was quiet. Luna hadn't said a word since she apologized. Not to Celestia or anypony at the party. It worried Celestia a bit, but she reminded herself she needed to be patient. Luna was probably tired out from the party, and still feeling bad about the past. Everything would be okay. Let Luna sleep and they could talk about everything when she was rested. Celestia was eager for everything to return to normal between her and Luna, but it wasn't fair to place expectations on Luna right now. Patience. If she could wait a thousand years, she could give Luna all the time she needed to feel better.
The carriage landed in front of the castle, and the sisters stepped off. Luna's gaze didn't move from the ground as she followed Celestia inside. It seemed the party hadn't been as much help as Celestia hoped. Maybe she could say something. She hadn't been able to talk much to Luna at the party because the other ponies crowded around Luna, but now that they were home, she could think of something to get Luna to talk even a little. A simple question was better than nothing.
"Oh, Luna, did you enjoy-" Celestia paused when she looked to her side to see Luna was no longer there. She turned around to see her sister wobbling on her hooves, trying to keep her eyes open. "Luna!" But in the two seconds it took for Celestia to hurry over to her, Luna collapsed onto the floor without a word spoken.
Panicked, but staying composed, Celestia ordered her guards to get the medical team. Less than two minutes later, Luna was being carried off to the castle's infirmary. Celestia followed, but of course, she had to wait outside of the treatment room. She paced back and forth, every grim possibility seeming to run through her mind. How could this happen? Not on the first day of Luna's return! They had just been reunited! No, this couldn't happen now! Luna had to be okay! She just had to!
"Princess?"
Celestia froze the instant she heard the voice, and she looked up to see one of the doctors addressing her.
"She's okay."
Celestia breathed a sigh of relief.
"She passed out from exhaustion. We've determined her power is very low. She must rest, but when she wakes up, please have her eat right away. Her blood sugar is too low."
Celestia understood. Two more doctors brought Luna out on a stretcher, and Celestia carefully hoisted Luna onto her back. She thanked the medical team for their help, and walked away to carry Luna to her bedroom.
When Celestia relocated from the Everfree Forest to Canterlot, she had Luna's room rebuilt. She gave no explanation for it. It was all she had left of Luna, and she would've hated herself until the end of time if she lost it, whether or not Luna returned. She wasn't happy her sister was ill, but she'd take that any day over never having Luna with her again. Plus, if she had to take care of Luna for a while, that meant she really could be a big sister again. She missed that.
Celestia tucked Luna into bed, but didn't leave her side. Instead, she rested by the bedside. She didn't have the heart to leave Luna right now. Luna slept soundlessly. She didn't snore and her breathing was barely audible. Celestia watched her, feeling an odd mix of relief and sadness. Every few minutes, she caressed Luna's mane or gave her a light kiss as she slept. It wasn't until Celestia took notice of the clock that she begrudgingly pulled herself away from Luna to go to the balcony and lower the sun.
She slowly descended the fiery orb from the sky and gave its place to the brilliantly white one. It didn't feel right for Celestia to raise and lower the moon now that Luna returned, despite Luna wasn't capable right now. Eventually, this task would be Luna's again. On the upside, at least Celestia could look at it without feeling grief since her sister's silhouette was no longer planted on it. She'd never quite gotten the hang of the stars, however. That was Luna's craft and she learned a long time ago it couldn't be mimicked.
Celestia hurried back to Luna's bedroom and rested by her bedside again. Luna hadn't moved an inch. Celestia lied her head on her hooves and allowed herself to fall asleep. She hoped Luna would be awake by morning. They could have breakfast together and finally talk. "One day at a time," she thought to herself. "Everything will be okay."