It's very important to take your cat to a veterinarian immediately. They can diagnose the problem and provide the right treatment.
Here are some possible causes of a weak and lethargic cat with a decreased appetite:
* Illness: Infections, parasites, kidney disease, diabetes, and many other illnesses can cause weakness and loss of appetite.
* Pain: A cat in pain might not want to move or eat.
* Dental problems: Tooth decay or gum disease can make eating difficult.
* Stress: A change in environment or routine can cause stress and lead to decreased appetite.
Please don't hesitate to get professional help for your cat. Early intervention can make a big difference in their health and well-being.