Luna had entirely withdrawn into herself after the fire. There was no reason to go outside the shop, no reason to risk the nightmare that was their current reality. Nothing to draw her from sanding wand after wand - each of them useless, as she did not have any where near the training needed to complete them. Nothing - but the fact that she'd finally run out of biscuits.
The dark was more safe than the light, Luna figured, after all - that was how they had managed the last year of hogwarts. Going out at night, replenishing supplies while the professors slept. Evil had a reason to prefer the dark Luna understood, it was easier to hide in the shadows.
So, she found her way to the little sad garden they hoped upon, wandering and wondering until Nevile was in front of her - praying perhaps? To the plants to grow.