The most probable possibilities are:
Broken back, no recovery
Bruised back, recovery might happen with confinement and rest
E. Cuniculi, recovery might happen with treatment
Poison, recovery might or might not happen
Look at the rabbit to determine whether it is in pain. Things like squinted eyes and grinding teeth can indicate pain. You can give up to one chewable baby aspirin twice a day for pain, or willow twigs. Or you may elect to put the kit down... especially if its condition worsens.
I would basically begin treatment for bruised back. I'd confine the rabbit to a small carrier (or a small Rubbermaid tote with decent depth), with food and water or electrolyte solution within easy reach. We have one our wry-neck (E. Cuniculi) rabbit lives in. It has the lid off during the day, with a bungee holding the door on, and gets closed at night. We put a makeshift wire floor in it, so he isn't in the bedding. You could also start treatment for E. cuniculi, but I don't know that you'd want to, since he's a meat bun. Might just confine him, limit movement, and see what happens.