What am I doing wrong?

So one day my tomato seedling was al fine strong stem, great green leaves and the next day this happens (check pictures) can any one help me plz? Ps: it's first the bad picture

Either time release/ added fertilizer in the potting soil, or the fiber pots got too dried out.
Those fiber pots work best in ribbed 10x20 trays , with just enough water to cover the bottom ribs and wick up to the plant.

Probably the soil mix, tell us what you use and what are you watering with.

Just tap water and compa sana soil but I am scared I give to much water

COMPO SANA® Universal Potting Soil
UNIQUE COMPOSITION FOR AN OPTIMAL PLANT GROWTH
The start fertilizer works immediately and provides an optimal nutrient supply for up to 6 weeks

There you go, get new soil, use that stuff for transplanting when they get bigger, maybe

I'd use different pots also, easier to control moisture levels for me.
I use those for Sunflowers, cucumbers and beans, big root growing crops.

So are my plants just go a die know

They are getting overwhelmed as soon as the root starts doing its thing, I had this exact thing happen twice in a row with vegetables germinating.
Same guy who sold me the original dirt and fiber cups, then sells me new seeds and correct soil mix and yells at me like I'm the idiot.

All his mulch disappeared from the storefront that night, strangely enough.

Thank you I feel sad not you foult though you helped me

So how old they got be for I can use this soil?