Leaves yellowed overnight, what's the problem? (Pics)

Woke up this morning to check my seedling and it seems the small round leaves have yellow and look shitty overnight also the progress of the seedling has stopped and not seen any improvement at all, not sure what the issue is would be nice if someone could help me figure it out?
Also have an other seedling which is doing perfect so don't see the issue

it's called, LOVING YOUR PLANTS TO DEATH, all noobs do it. seriously, its fine, go away and let it do it's thing, it doesn't need much from you right now.

The small round leaves are cotyledons. They are the food for your seedling so as the plant grows, it's natural for them to wither away

Yeah ano but it just seems a problem when growth stops while the other powers on and the leaves change colour over night from healthy green to bright yellow

I think it is okay to be sad about them. It is like when your dog dies. Except that your dog only dies once and the cot leaves will die on every plant you ever grow. So maybe a little different, but I respect your right to feel really horrible about it. I will now have another toke in honor of your fallen cot leaves.