I had the best luck getting my rabbits eating again after their surgery, by offering their usual leafy greens. If by the next morning a rabbit isn't eating anything or very much, that's usually when I'll start syringe feeding. If you aren't familiar with syringe feeding, it would be best to have your vet show how it's done, as doing improperly and too quickly, could pose a risk of aspiration occurring, which can threaten the rabbits life if it does occur.
https://rabbitwelfare.co.uk/rabbit-health/medical/syringe-feeding-guide/
https://wabbitwiki.com/wiki/Medicating_your_rabbit
It's also important to keep your rabbit warm following surgery, as they will usually have a hard time regulating body temp following anesthesia. I've used a cotton bag filled with rice, and microwaved to warm up(make sure it's not too hot) and then place the warmed pack by my rabbit. Chew proof pet heating mats are also an option.
Keep an eye on chewing at and messing with the incision site as well. I set up a small recovery pen next to my couch, to keep an eye on them for that first day or two. Then they need a small enough area to prevent running and jumping up, 1 week for males, 2 weeks for females, to prevent causing any internal damage due to having their surgery.
https://wabbitwiki.com/wiki/Spaying_and_neutering_rabbits
Pre- and Post-operative care of Domestic Rabbits
If at any point, you're concerned about your rabbit having problems eating or not doing well, you should contact your vet immediately.
