WHY?

The seedling disease is a common fungal problem that mainly affects seedlings. The cause is usually a Pythium, Botrytis or Fusarium fungus (more on that later). Whether these fungi are in the soil or have entered your grow room/garden through another plant, they affect the stems of young seedlings. As a result, they become limp and die within 24 hours.

I’ve never used peat plugs or jiffy plugs like this but seen pics just like this so often.. I’d recommend a diff starter medium personally. Especially going into what looks like hydro.. go rockwool

Nowhere near enough light. Also, do not place the jiffies in any sort of container or "humidity dome", they do not need it and will sprout just fine uncovered. However, they will need direct sunlight or strong electric light once they come to the surface. Reflected ambient light from sitting on a window sill is not enough. All is not lost, just replant them, bury them up to just below the round leaves (cotyledons) and place under strong light and they might still be fine. Hope this helps, Organoman.