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 Old Dog's Back Leg Limping & Collapsing: 6 Reasons
My Dog is staggering and falling over what should I do?
3 Simple Ways to Strengthen an Old Dog's Hind Legs - wikiHow
My dog suddenly couldn't walk on her hind legs. She seems fine now. What's wrong? - Quora
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Help! Old Dog Back Legs Collapsing