A cat's weakness, vomiting, and inability to stand up may be caused by a variety of reasons. Here are some possible causes and suggestions:

Poisoning: Possibly ingesting poisonous substances. Contact your veterinarian immediately.

Digestive system problems: such as acute gastroenteritis, pancreatitis, etc. Watch your cat for other symptoms.

Infection: Viral or bacterial infection.

Endocrine diseases: such as diabetes.

Nervous system problems: brain diseases, etc.

Malnutrition: Long-term improper diet or nutritional deficiency.

Stress response: such as environmental changes, being frightened, etc.