Most of the time, as you will know if you own a rabbit, they rarely stop eating. That means that, whenever they do, you will notice a big change. The problem is, that when they do stop eating, there is usually a reason for it, and rarely a non-serious one. The best thing to do when you notice that they have stopped eating is to take them straight to the vets, and one that is good with rabbits. There are various reasons why a rabbit may stop eating, for example, stress from loud noises that they are not used to, teeth problems, and general pain.
If you know that stress is the cause for your rabbit not eating, then you should do what you can to remove the cause as best as possible. Alternatively, move the rabbit away from the cause. If you are unsure of the reason, and there are no environmental changes, then you should take them to the vets. There is a disease called gut stasis, and it can be severe, even life-threatening. That is when the movement of the gut that pushes food through the intestines, stops. Furthermore, that can lead to the digesting food becoming dry and unable to pass. Make sure you get your rabbit checked as soon as possible.