Seedling fallen down

As @El_Andy says, it's just the stem is not quite able to support the weight. I had exactly the same thing happen to my Orange Sherbet in week 1 or 2, caused by lack of fresh air. Seedlings want it humid, but you still need some air movement. Just prop it up with whatever you have to hand (I twisted up a support out of garden tie, you can check my pics), get some air around it (not so much it blows it back over!) and it will bounce back. Mine went on to be the biggest plant after I thought it was going to die. A light breeze also helps seedlings develop stronger branches - the swaying motion from the breeze causes small micro-fractures in the plant's cellular structure, which then 'harden up' in response. The branches will then support bigger colas later in life and be less likely to need support. Andy's comments about the light are also correct - if your seedling wants more light, it will stretch - just bring your lights down a little closer if it still looks a little leggy after getting back on its feet. You shouldn't have anything to worry about. Aside from being a little.... horizontal, it looks perfectly healthy and no reason it won't go on to be a thriving beast. Best of luck, man.

Most Likely just weak stem development because there’s no passive air movement. You don’t want a fan pointed directly at it, or any violent thrashing movements, but allow the seedling some lateral movement via light air movement. Right now you can take a paper clip or a toothpick etc. and use it to straighten up the seedling. It’s just top heavy with a weak stem. Use a garbage tie or create a loop around the stem by careful shaping the paper clip around the stem. Additionally, if your light is too far away from the seedling it will keep stretching towards the light. Not sure if that’s contributing here but keep that in mind as you stand up the seedling again. Good luck.