This is not the same as a puppy still learning
A dog who has been reliable for weeks or months and then suddenly starts having accidents is showing a change, not a gap in training. Changes deserve investigation before retraining.
Medical causes are common
Urinary tract infections, bladder stones, diabetes, kidney disease, and age-related changes can all cause sudden accidents, sometimes before any other symptom is obvious.
Stress and environment can also play a role
A new pet, a move, a change in schedule, or a stressful event can sometimes affect a previously reliable dog — but this is worth discussing with your veterinarian alongside a medical check, not assumed on its own.
What to avoid
Keep the response gentle.
Avoid treating a sudden regression as defiance or a training failure — it is a signal to investigate, not a reason to punish.
Safety boundary
When to get more help.
Any sudden house training regression in a previously reliable dog warrants a veterinary visit before anything else.