Sounds like classic damping off. Let them dry as much as possible to see how many you might save and then repot as soon as possible. You should leave them uncovered once they come up, probably the cause of them damping off.

Never cover the plants once they germinate. Covers are for germination only and aren't even really needed for that. After germination they only do harm, no good. And if any of those survive transplant them deep burying all that bare weak stem in the soil.

Dave

Yes, definitely take the cover off the moment you get germination. Those covers are only used to keep the humidity and heat in until the seeds sprout, then are to be removed immediately. I also worry that by putting them covered into direct sun, you may have cooked your poor seedlings. The heat would really build up in sunlight.

Are they really three week old? They look much younger than that to me. My three week old seedlings are starting on their second set of true leaves.

I'm not sure you'll be able to save these. You might want to start some new seeds, just in case. Good luck!

That's frustrating news, but thank you for the input. That's certainly the last time I follow the directions on the tin.

Kathy, they're between two and three weeks. The seedlings that I planted in a pot were sown a week earlier and the second set of true leaves is coming in now. I probably should have realized when I saw that, that there was a problem.

Would it be worth starting new seeds this late in zone six? I probably wouldn't be able to transplant them until June.

Never found these peat round any good. Just canôt keep it right moisture and everything. Junk IMHO
Here is what I would do if you have extra seeds, soak them for few hours in somewhat warm water, not hot but just lukewarm. Put them into paper towel and into plastic bag, leave it somewhere where it is fairly warm. It will germinate within day or two. Then transplant into small cell or small cups, keep it under lights for 16 plus hrs and give bottom heat, very gentle for about a week, make sure it does not dry up. Keep it covered only for first day until you see cotyledons showing up. Remove the plastic top after. Make sure there is gentle breeze from air duct, fan or something going once in a while. It is no good to put it in direct air flow but gentle air movement is a must. Do not use same trays or wash them really really good. What I am leading to is that with extra good conditions and coddling it is possible to speed up process of growing seedlings even if you are being on late side of starting. Just like you see in the garden nothing seems to be blooming and growing, then give it couple warm sunny days and things are going gangbusters.