Whether your feline friend has ceased eating, begun overeating, or is avoiding using the litter box, they might be experiencing some sort of depression.
In order to help solve the problem, you must get to the bottom of what is causing these feelings and emotions for your kitty. Think about any crucial changes in their life such as a new pet they feel the need to compete with or a family member missing from home. Other changes such as a new kind of food or litter can also affect your cat. Although these may not seem life-changing to you, they can be detrimental to your cat's mood.
In some cases, it may just take extra tender loving care to solve your feline's issue. Try putting aside extra time each day to groom, pet, and play with your pet. Remember that some cats don't need much attention but others require just as much as dogs, or even more! They appreciate physical affection and interactive toys to keep them busy, so make sure to cuddle and supply scratching posts or a mouse-on-string to keep active.
Although most cats live indoors, the outdoors can be really mood-improving. A harness & leash, window perch, or catio (cat patio) can allow your cat to experience fresh air and sunshine without the worry of any predators sneaking up on your furry friend.
If spending more time with your kitty doesn't cheer them up, it may take some more work. A cat who seems sad may actually be experiencing pain, discomfort, lack of energy, or other difficulties from an illness or injury. If none of the previous solutions seem to help, your vet may even recommend antidepressant medication for cats whose mood causes exceptional behavioral problems.