This just happened to one of my rabbits, Ferkel. I had noticed she hadn't pooped in 24 hours and was freaking out to say the least. She was still eating, which is a good sign. Is your bun still eating? That's very important. Here's what I did that worked:
Sonce she was eating i didnt rush er to emergency, but the rabbit vet in my area never has opens until a week later, sp i coukdnt get er an appointment. I read online about some things that might help. I flipped her over gently and gave her stomach and entire belly area a gentle massage. Just rubbing the stomach very gently, make sure not to press too hard. No clue what I was doing, but as I got closer to her rear I heard some gas bubbles come out. I set her on the floor and within a few minutes she popped a nice perfect pile of droppings. Another 12 hours went by and no more droppings from her, so I repeated the process. Again, same results. Then another 6-10 or so hours without pooping on her own and again I massaged her stomach and she pooped. After that I gave her subq fluids and the next morning I woke up to find droppings in her litter box. I think a combination of the massaging and especially the extra hydration really helped. Now Ferkel is back to normal, which I am SO happy about.
Let me know how it goes! I would definitely get your bun to a rabbit savvy vet if you can. In the meantime try a massage or some extra fluids.