plant death diagnosis

i had a plant 2-3 weeks old that was growing perfectly fine. it was growing outside.i went to water it yesterday to find it leaning all of the way over on the ground. its dead today. the stem looked like it cracked in the middle. help please! im about to plant more but i dont want the same problem to occur

Was it two to four leaves but very tall with a very thin stalk?
If so it stretched itself to death... over watering can speed death along when that happens. This is why most of us sprout our seedlings indoors with the light as close to them as possible. Otherwise the seedling will continue to stretch towards the light source (or lack there of) until it exhausts it's nutrients and just keels over.
If the stem actually (cracked) I've got nothing for ya... only thing I can think of there is some sort of pest.