seedling stem shrinking and turning dark where leaves start

seedling
day before
Attachment 292884

suddenly yesterday morning
Attachment 292885Attachment 292886

have most basic setup with natural sunlight and lil' pot with planting soil in it + watering without feeding.
what troubles me is that "just" happened.
have two plants, germinated them same time. put them into medium at the same time. one was bigger and this one on pictures was lil smaller.
but it grow faster and before that damn dry up almost same size on even lil bigger than other plant.

right now it looks like under watered
Attachment 292887

is there any way to save it? or its lost?

it's stretching for more light. can you put it in a sunnier space somewhere or give it 18hr/day of artificial light from a cfl just to get it over the hump?

Soil also appears over saturated?

Peace

@ when firs picture was made soil was ok. i was worried i overwatered. my girl came and checked the soil and screamed at me that soil is too dry... at that moment leaves were pointing a lil down - like a plant needing a water. so i watered. next morning boom plants top is facing down the soil and leaves had dried - even the newgrowth had marks. used the paper clip and pen to turne the leaves up and hold the pot under the sun (moved pot even when sun moved). today plant looks like its tottaly dry. stem seems healty, seems that these dark shrinks wont let juice through and top dries out.