Resurrecting a dead fish is impossible and should not be attempted. However, a Goldfish appearing lifeless or in extreme distress might still have a fighting chance. Immediate changing of the water, adjusting its parameters, adding aquarium salt or introducing appropriate medication can sometimes improve their condition.
But prevention is always better than cure. Ensuring optimal water conditions, providing a balanced diet, and paying attention to any changes in their behavior can drastically increase the likelihood of healthy, vibrant Goldfish in your tank.