It is important to try to get the rabbit to eat at least a little something as this keeps the gut moving and digestive tract working. Some rabbits will stop eating for a day or two and pick back up eating as if nothing happened. This is particularly true of animals in a molt. Some take a molt hard, especially older animals, bucks in particular have been my experience.
When a rabbit is molting I give them a papaya tablet once a day. If they aren't eating well, I up it to two tablets a day. The papaya enzyme helps breaks down fur blockage, is good for the digestive track and is sometimes the only thing they appear interested in. (I also give a papaya tablet to does that have kindled or done a lot of fur pulling for nest building).
Also very important is hay. A rabbit may stop eating grain but almost always eats some amount of hay. This is good, that too keeps the gut moving and hopefully will catch some hair on the way out. If they are at least drinking water and eating hay I don't worry very much. A tablespoon of oatmeal on top of the grain many rabbits enjoy and it is good for them.
Why is it important to keep the gut moving? It prevents blockage that can lead to bloat. A rabbit with blockage won't be hungry as it feels full all the time.
Okay, so water, hay, a papaya tablet, what else? There are other things that might perk up a rabbit's appetite. I am talking about "treats". Treats are things that you don't necessarily give every day. Generally the word on treats is moderation. If a treat isn't gone in less than a few minutes, you've given the rabbit too big a treat. Appetizing treats may be: a 1/4 slice of bread, an inch slice of banana or apple, a strawberry, a few potato peelings, a 1/3 of a carrot, parsley or a teaspoon of non-sugary yogurt.
If they aren't eating anything and are getting very thin you can try a product called Nutri-Cal. You can purchase in feed or pet stores or online. This is a high energy sticky paste the color of honey in a tube. You can try putting some on your finger and feeding it to your rabbit. This is best done while holding the rabbit upright in your arms or slightly on it's back. Don't try to feed it by simply sticking it under their nose. They don't like their whiskers touched and will likely back away as they think they are bumping into something. Some rabbits won't eat while being held so you can try putting it on the rim of the food dish. Another trick is putting a small amount, like the size of toothpaste you'd put on a toothbrush, on the inside each front paws. The rabbit will lick the Nutri-Cal to clean it off it's paws.
IMO, feeding twice a day instead of once a day is an advantage to noticing when a rabbit stops eating. If not feeding twice a day, look in on every bunny with a second visit to the rabbitry. If they are Hollands and most other breeds, they are all clamoring for the attention if not for the food!