A spay is a major surgery. So she's going to be uncomfortable, even with adequate pain meds. Some rabbits can also have some nausea from the anesthesia.
I've had the best luck getting them eating again, by continuing to offer some of their usual leafy greens, every hour. If she will at least eat a small amount each time, this will help keep her digestion moving and also help keep her hydrated.
One very important aspect of a rabbits post surgical recovery, is making sure they have a warm body temp. Anesthesia can lower a rabbits body temp, and they can become hypothermic after surgery. A cold rabbit will be less inclined to eat, so it's important to monitor the body temp and warm the rabbit up if needed. Easiest way to check if a rabbit is cold, is by feeling their ears. If the ears are warm, the rabbit is warm, if the ears feel icy cold, the rabbit is too cold and needs help getting warmed up using a safe warming method.
If your rabbit isn't eating semi normally by morning, you'll need to contact your vet for a syringe feeding mix and instructions on syringe feeding. A rabbit shouldn't go more than 24 hours without eating an adequate amount to keep their digestion moving well, or GI stasis can set in.
http://www.bio.miami.edu/hare/opcare.html