Reasons for sudden lameness of French Dou's hind legs:

1. Neuromuscular damage caused by trauma, resulting in hind leg movement disorder and weakness.

2. It may be caused by certain metabolic diseases. Kidney failure can also cause sudden lameness of the hind legs.

3. Neuromuscular diseases can also cause this condition, such as myasthenia gravis.

4. It may be caused by immune-mediated diseases, such as immune-mediated arthritis.

5. It is also possible that the French Bulldog's hind legs suddenly became lame due to calcium deficiency.

Solutions to the sudden lameness of the French Bulldog's hind legs:

1. For the sudden lameness of the hind legs caused by calcium deficiency, the owner needs to give the pet special calcium tablets. Or calcium-rich food supplements.

2. If it is caused by an injury to the hind leg, if it is a trauma, just use normal saline, iodophor, and anti-inflammatory drugs to treat the wound. If the injury is internal, surgical repair will be required.

3. If it is caused by a disease, it should be treated with appropriate medicines according to the disease.

4. If it is caused by an immune-mediated disease, the French Dou should be given anti-inflammatory drugs, and then immediately cooperate with the doctor for examination and treatment.