It's damping off from the looks of things. It's dead meat it will not bounce back or survive at all. You killed it because you got water on the stem at the base where the stem and soil meet. You never ever touch water to the stem of a tender seedling because there is pathogens in the soil that will get a foothold on your soft delicate seedling tissue and destroy it. You always water very carefully around the perimeter of the stem, but never directly on it. I know you think it's not gonna reach the tap root, but soil is like a sponge and it will wick into the soil and distribute. You only need water in the general area of the tap root you don't need to douche the whole body of soil. Less is always more with weed in the beginning.