They probably were damped off. A common fungus, if given the right conditions, attacks the young plants and causes them to die. Once this happens there is no cure, you have to try and prevent it from happening. You can find a good explanation here:

http://www.extension.umn.edu/garden/yard-garden/diseases/damping-off/

Other sites can give guidance on how to avoid this problem. I lost a tray of papaya to it last summer. I was really bummed. Best of luck.

thank you!