Summarize this article:

Not all rabbits are friendly. But if your rabbit usually leaps around and greets, but suddenly ceases to do so, this is an indication that something might be wrong. Watch for signs that your rabbit is less mobile than usual, such as hunching over or limping. If your rabbit is not eating normally, it might be ill. Watch for leftover food from the last feeding.  Also, monitor its droppings.  If there are no droppings in the litterbox this indicates that the rabbit is not eating. Be attentive to the size and shape of the droppings. Ideally, the droppings should be large and round. If they are small, irregular, or liquid, it is possible that your rabbit is sick. Your rabbit will often make a soft, grinding sound with its teeth when it is happy. However, if the sound is louder than normal it could be a bad sign. Often this is an indication that your rabbit is in pain. Begin by offering your rabbit its favorite treat. If it refuses to eat the treat, it might be sick. Move on to testing the rabbit’s temperature. If it is well, the temperature should be between 101⁰F-103⁰F.  You should ask your veterinarian to demonstrate how to take your rabbit’s temperature. If you do this in advance of any signs of illness, you will be prepared in case of an emergency. To take your bunny’s temperature you will want to place it on its back, either on a padded surface or your lap. Hold your rabbit’s head and shoulder against your stomach, so that its back curls into a “C” shape. Secure the rabbit’s back legs so that it does not kick. Once it is calm insert a lubricated plastic thermometer no more than one inch into its rectum. Be sure that the rabbit is well restrained and is not able to move while you take its temperature.  Do your best to cool your rabbit off if it has a high fever by putting cold objects up against its ears until the temperature drops to less than 104 degrees.

Summary:
Watch for changes in behavior. Observe eating habits. Listen for tooth grinding. Test for signs of illness.