If your dog can't walk on it's back leg suddenly, it's most likely due to injury. But sudden lameness can also indicate a neurological issue.
Why Is My Dog Limping? 6 Common Reasons
Sudden Paralysis in Dogs: Causes and Treatments
13 Causes of Limping in Dogs - PetHelpful
Why is my dog unable to stand?
How to Tell if a Dog Is in Pain
5 Causes For Sudden Limping In The Back Leg Of Dogs
Hind Leg Weakness In Dogs – Signs And Symptoms – Vital Vet
Why Is My Old Dog's Back Leg Limping & Collapsing: 6 Reasons
Why is My Dog Limping Suddenly?