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:

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