Seedlings under the sun

Try sprinkling a little cinnamon around her and water in. If it isn't to late you could save her...Cinnamon prevents and cures damping off

I'm guessing they were either in a window or under a light, Direct sunlight is POWERFUL, You need to "Harden off" your plants, especially if its hot and direct sunlight. Start outdoors with 2h shade, Then the next day 3h shade, then 4h shade with 1h morning or evening sun, Never blast them with high noon sun until they have had 3-4h to morning/evening sun with noon shade. Damping off they shrivel on the stem but the leaves look normal for a day or so. This is heat/sunburn damage.

Could it really be the sun's heat or did it trigger the damping off? during that time, rate of growth for all microbes in soil increased, but no... not just because it was in the sunlight. small chance it coincidentally occurred at that moment, but since you also mention the recovery is happening, i'm even more confident that's just light-related. they don't recover from dampening off. becareful with h2o2... and if in soil, i'd only use it when absolutely necessary... it has no prejudice, and your microbes are important for soil grows. use a bit more crushed leca balls or perlite etc... things equivalent to that should be 50% of your substrate for best oxygenation / drainage / healthy rhizosphere. while i don't beleive this is damping off, it sees like your substrate holds a lot of water, just a guess. that is an opportunity for it to occur more often. sometimes it just comes with the soil/substrate and nothing we can do, too.

Start a fresh as they have dampened off. Soil was most likely to wet. Next time start them off in a better container then that. Try a solo cup, put some holes in the bottom put soil in wet soil then put seed in. When it comes to move to a bigger container its easy to get it out the solo cup.

I am not convinced that it is damping off. I didn't gave them a lot of water. I used a spray with part water and a bit of hydrogen peroxide. The soil is a mix of 1 part peat, 1 part sand, 1 part potting soil, 1 part crushed expanded clay. They just got this way after getting under the sun. Now that they are back in the shade looks like they are recovering. Could it really be the sun's heat or did it trigger the damping off?

if I see it right it's a paper egg cup,she gave them a lot of water...unfortunately these seedlings died.I suggest that you start the seedlings in larger pots because the pots you use are very tiny.

That's damping off, a fungal disease. It happens when you let the medium get too wet. Unfortunatelly once it sets there's not much you can do to save seedlings. Take all the dead seedlings out, clean the pots and the whole place because it spreads very fast, before you try to start again. Also aerate the medium, a good one for new growers is 50-50 peat perlite. Also watch how much water you give them, they don't need much at first.