If your German Shepherd suddenly cannot walk on his back legs, this could be a sign of a serious medical condition, and it is important to seek veterinary care immediately. There are several possible causes of sudden hind limb paralysis or weakness in dogs, including:
Intervertebral disc disease (IVDD): This is a common condition in dogs that affects the spinal cord and can cause sudden paralysis or weakness of the hind limbs. This is more common in certain breeds, such as Dachshunds, but can occur in any breed.
Degenerative myelopathy: This is a progressive degenerative disease that affects the spinal cord and can cause weakness or paralysis of the hind limbs.
Trauma: A sudden injury, such as a fall or hit by a car, can cause paralysis or weakness of the hind limbs.
Infection or inflammation: Certain infections or inflammatory conditions can affect the nervous system and cause paralysis or weakness of the hind limbs.
Tumor: A tumor or mass pressing on the spinal cord can cause hind limb paralysis or weakness.
Your veterinarian will need to perform a thorough physical exam and may recommend diagnostic tests, such as X-rays, blood work, or an MRI, to determine the cause of your dog's sudden paralysis or weakness. Treatment will depend on the underlying cause and may include medication, surgery, or physical therapy.
It is important to seek veterinary care as soon as possible, as some of these conditions can progress rapidly and cause permanent damage if left untreated
Here in England I see owners of dogs with same problem put their back & legs on a two wheal little platform. as long as your Dog can use front legs, mobility again is possible. Look for a suitable item in children[s toys for this job...good luck...
Perhaps it must be due to a spinal injury preferably in the lumbar/sacral region.... sometimes due to falling or hitting with any hard object in the lumbosacral region of the animal. Consult any nearby vet