Bended seedling why?

At some point it wasn't getting enough light and it stretched and got weak, then fell over. Becareful about how you react... if the stretch occured early on and you are now providing more light, there's nothing you can do but prop it up -- use a toothpick or whatever to hold it upright. Adding a bunch more light can cause more problems. The node spacing on recent growth seems fine, so i assum this happened closer to when it initially sprouted. so, prop it up.. keep letting new growth node spacing guide light intensity and it'll toughen up. If you pot up, you can bury the stem a little bit, but don't over-do this either. You can bury some stem but a long stretch of stem buried is a vector for pathogens - wet stem deprived of light... it will slowly differentiate into root, but the risk of infection is real. I'd suggest using size-appropriate pots throughout a plant's life. Potting up does not 'shock' plants if done right, so it's even best for autoflowers too. It's difficult to properly water a tiny plant in a large pot. Tend to train superficial roots due to partial watering or it stays wet so long the water stagnates in the pot. Just a bunch of risk for no reason. The ease of watering alone is enough to avoid large pots, tiny roots.