Why’s my seedling droopy and yellow? And how to fix it?

Yeah, lack of light and fan for strength. The window grown plants I've seen are lanky and not vigorous. The cotyledon provide the energy for root growth and first leaves. Mine never yellow in my grows so suspicious to me although any stress can affect the health. Falling over probably drained her energy, shocked her. Is your soil pre fertilized? I use Fox Farm or Roots Organic that feeds them for 3-4 weeks. Every issue is not always nutrient related. Lack of proper light, heat, humidity and over watering are your biggest enemy at this stage of life

Just the cotys are yellow, so i wouldn't react yet. If the lowest serated leaf remains properly pigmented, it's just shedding the cotys and no big deal. It got lanky due to lack of light, so you need to providee more light per day. Looks like you did that based on top couple growth nodes... Don't go too far the opposite way. If growth nodes don't have some healthy spacing it's a sign it needs a little less.

Not enough light probably and no air vibrations to harden the stem, those are my guesses. Maybe get yourself an extra light for support during dark days, and definitely get yourself small fan.🤞🍀♥️