Rabbits are really fragile and sensitive creatures. Some rabbits will stop eating when they are emotionally stressed or depressed. Loud noises, sudden movements or even a change in its diet and routine can stress a rabbit out.
It is very dangerous when a rabbit does not eat because they could develop gut stasis. This occurs when there is a slowing down in the intestines' movement. When this happens, gas can build up and cause a painful bloat. This often results in a loss of appetite and intense discomfort and pain. In such cases, it is very important that medical attention be given to your rabbit immediately.
Another common disease is dental malocclusions. This happens when the teeth on the upper and lower jaw are not aligned properly, resulting in overgrown teeth and gum/tongue injury. When the teeth overgrows, the rabbit cannot eat and swallow its food properly. There will be signs of pain, teeth biting, drooling (with a lot of saliva caked at the chin and neck area) and ill thrift. A medical checkup is needed to confirm the severity of the dental problem and most often, dental trimming and filing will be required.
Other causes of loss of appetite can include skin infection, urinary bladder infection and chest infection. If you observe that your rabbit has not been eating for more than 18-24 hours, bring it to the veterinarian immediately.