Answer: Usually, seedlings that flop are in need of more direct bright light. Or, they may be overfertilized especially with nitrogen. Too, if the plant you are growing is a vining plant it may be following its normal growth habit.
If your seedlings are indoors, a window sill is not usually enough light. You should give them supplemental light such as ordinary fluorescent bulbs kept just an inch or two above the foliage. If your seedlings are elongated and flopping over you might need to start over, being careful to give them direct bright light as soon as they being to come up through the soil.
I hope this helps you trouble shoot.
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