same thing happened to me my 1st time. In my case I think it was a combination of 4 things. 1) Light too far away causing stretching...creating 2) a top-heavy seedling...3) high heat, making the young stem buckle, and 4) a fan finally blowing it over.

I staked it with a matchstick (head removed) and loosely tied it until I saw that it could stand on its own again.