How is your bunny doing now?
Rabbits often eat strange things. Most come out fine. There are cases that don’t have a good outcome though, and yarn would definitely be a concern. A vet wouldn’t do much to begin with, because a bunny can’t vomit and you can’t induce vomiting in a bunny due to their anatomy. So what comes in must pass through. In very rare cases surgery to remove a blockage is needed. But you’d first try fluids, by mouth, and hay because of it’s high fiber content. Wet greens, since your bun won’t eat hay, although hand-feeding as Sarah suggests can sometimes make a bunny willing to try at least a strand or two. If the bunny loses it’s appetite and/or stops pooping, it’s an emergency that requires vet care. With a blockage, treatment is based on hydration. Motility drugs are not routinely used, because increased intestinal movement can cause the intestines to rupture if there’s a blockage.