This happens when the light is too far away from the seedling and the temps are too high. Could be one, the other, or both. A lack of wind is also the problem.
This is what I do when this happens to me: I stake the plant to a tooth pick or shishkabob skewer using soft garden tie or twist tie, I make sure that it is not going to fall back over again and that the tie is affixed LOOSELY around the stem, so that it does not choke the plant. I usually wrap one end of the twist tie around the toothpick several times so it looks like a flag, and then do the same to the stalk of the seedling with the other end.
Now we have our plant standing tall and proud, but she is still a wimp, so we put a gentle breeze on her with a fan on low so that she is dancing, but not rattling around. As long as your plants dance they will grow strong and stout, which is what we want in our plants. The bending from the wind creates micro tears in the stalk and as it heals itself, it thickens up to protect itself.
Problem Solved.