The celery in my garden turned brown...

Your plants were probably infested with blackheart, a disease caused by a calcium deficiency when the plants are young. Check your soil pH. If it’s 6.0 or lower, that’s the likely culprit. Add lime to increase the pH to 6.5 to 6.8. If you think you already have enough calcium, weather conditions may have prevented the celery from absorbing it. Extreme heat, extremes of wet and then dry soil conditions, or high salt in your soil also may cause the condition.