Suddenly one of Teddy's hind legs cannot touch the ground. It may be caused by trauma, fracture, long-term calcium deficiency, joint problems, or hip dysplasia. It is best to check whether there are sharp points on the dog's feet first. Prick and then touch the dog's leg to see if there is any pain reaction. If the dog continues to have this condition, it is best to send it to a pet hospital for examination and treatment to avoid delaying the condition and affecting the dog's normal walking.