I am sorry to hear you are worried about your cat. I am a veterinarian, and will do my best to help.
Can she move her legs? It is all four legs she can't stand on, or just the back legs?
Is she eating, and drinking still?
There may be hope for her, but it depends on finding the cause of her problem. She could have a blood clot in her back legs, and that can be treated sometimes, but usually indicates the cat has heart disease, or high blood pressure that also needs to be treated.
She could be paralyzed from a disc problem or injury. Does she go outside; could she have been injured? Some disc problems get better with time and rest, some need surgery.
There is a problem called tick paralysis, where a cat or dog loses the use of all the limbs and can only lie down on their sides. It gets completely better when you find and remove the tick. It is only seen in certain parts of the country, depending on the kind of ticks you have.
So the best place to start would be a good physical exam, to determine if this is a neurologic, circulatory, or orthopedic problem. She may or may not improve, but you have to get a diagnosis first, and a physical exam of her, testing reflexes, feeling her pulses, listening to her heart, etc. is where I would start if she were mine.
Let me know what else I can help with or answer.