A cat suddenly unable to stand up and lying on its side may be caused by hypoglycemia, long-term calcium deficiency, fractures, joint diseases, heat stroke, feline plague and other reasons. First, observe the cat's condition. If the cat is in low spirits, has vomiting, diarrhea, or is unable to stand, it is best to send it to the pet hospital for examination immediately. If there are no abnormalities, you can feed the cat a little water and then some liquid calcium.