A dog's sudden inability to stand up on its hind legs may be caused by calcium deficiency, injury or other diseases. Pay attention to adjusting the dog's diet. You can feed the dog's milk powder, egg yolk, beef and other foods to help the dog's body replenish nutrients. Properly take the dog to exercise and bask in the sun to promote calcium absorption. If it is caused by injury or disease, take the dog to the pet hospital for examination and treatment immediately to avoid aggravation of the condition and affecting the dog's health.