Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your girl is in pain, refusing to eat, and seems unable to use her rear legs properly.
Has she passed any urine or stool?
Is she drinking water?
If you pinch her toes on her rear feet can she feel that?
She may not be eliminating because she is unable to get into position, or hold herself in position, to go, or if she cannot feel her back legs then she may also not be able to feel a full bladder or have the ability to urinate on her own.
And if she is very painul, and/or unable to pass urine or stool she may not feel hungry, her focus is elsewhere.
Your pup may have one of a few things occurring.
A physical examination by her veterinarian and diagnostics such as radiographs of her heart and spine, an ultrasound of her heart, as well as cultures of her urine or blood and a complete blood count may help narrow things down.
If she goes out unsupervised then trauma to her spine is a possibility but it should leave some external signs, which you don't mention.
Another possibility is diskospondylitis, a bacterial or fungal infection of the intervertebral disks and surrounding vertebrae. It is usually caused by an infection somewhere else that has been spread to the vertebrae by the blood supply. It is a painful condition. Treatment is usually antibiotics or antifungals for at least 8 to 12 weeks, sometimes as long as a year. Some of these dogs require surgery to remove as much of the infection as possible and possibly stabilize the spine. Frequent re-evaluations by her veterinarian are necessary. Prognosis depends upon the amount of damage done and the organism causing the infection.
Intervertebral Disc Disease happens when the spongy disc or disc material between the vertebrae slips up and compresses the spinal cord causing pain and weakness or paralysis. It can be treated with anti-inflammatories and rest or surgery depending upon how much function is lost and how much pain the patient is in. Prognosis is much better if the patient's sense of deep pain remains intact and if surgery is done quickly in the cases that require it. It does not usually cause a fever.
Fibrocartilaginous Emboli is a piece of a degenerated intervertebral disc that has broken off and lodged in a small artery or vein near the spinal cord blocking the blood supply to her spinal cord and degeneration of the cord. It is often one sided, only painful the first few minutes to hours, and doesn't cause a fever. Signs don't progress after 24 hours unless another emboli is thrown. Prognosis depends upon how much damage is done initially. Recovery is slow and gradual, the most improvement happens between day 21 and 42, and may not happen at all if his sense of pain perception is lost.
Another possible cause of her condition is an Aortic Thromboembolism (saddle thrombus). This is a blood clot that formed in the heart, usually secondary to a primary heart disease, and has lodged in her aorta usually cutting off the circulation to her rear legs. This condition is very painful. It has a higher incidence in cats but dogs can be affected too. Temperature of the affected limbs is sub-normal. Her pads on her rear feet may feel cooler and look darker in color compared to her front foot pads due to poor circulation. Treatment is anti-clotting medication like heparin or low dose aspirin as well as pain medication such as Buprenorphine and treatment of the primary heart disease that predisposed her to developing a clot. Prognosis is very poor because of the damage done by the clot as well as the underlying heart disease. Recovery may take weeks.
Finally a spinal cord tumor or a tumor of the tissues around the spinal cord leading to compression of the cord and loss of function is possible too.
Your girl needs hands on veterinary care as soon as possible. I know it may be difficult to get her there, but often by slipping a blanket under her and picking her up that way allows her to be moved in a more comfortable manner.
This isn't something that I would try to deal with at home.
Please let me know if you have any further questions.