Normally that is the result of inadequate lighting. The seedling stretch, the stems become long and thin and they lean, can't support the weight of the small amount of foliage and possibly end up lying on the soil.
You can replant then deeper, leaving about 1/2 inch of plant above the soil. New roots will push out of what it now stem so your root system will develop faster. Add a small fan to blow on your seedlings. The swaying lateral and back and forth motion will cause them to grow thicker stronger stems, the very same way wind in nature causes trees to grow larger stronger trunks. And it is likely that you need to increase your lighting and or lower your lighting closer to your plants.
What lighting are you using, type, wattage, if CFLs, how many and whats the wattage? How high above the top of your plants is it?