A common mistake made when starting seedlings is to over-water them. This can result in something called damping off.
You know you have fallen prey to over-watering if your otherwise healthy seedlings fall over right at the soil level and the rest of the plant stem is strong.
It is commonly said that there is nothing you can do for seedlings that succomb to damping-off. Just chalk it up as a learning experience and plant new seeds in their place. However, the very lettuce seedlings you see in the photo on this page was revived by an antifungal solution comprised simply of pressed garlic and tapwater. ( Read: Garlic cures damping off in seedlings )
Here is a photo of a seedling that has fallen prey to “damping off.”Planting, seedlings break at soil, gardening and planting, Damping off