Google 'damping off'. That's what it sounds like. And no, they won't resprout. It sounds as if you had them too wet and too warm. If not do we're there any slug trails? Snails and slugs can mow down seedlings in a night and they're not unheard of indoors.

I posted a thread with the same problem a week or so ago. I'd never heard of damping off before but when looked it up it fit exactly what I was seeing. And sounds a lot like this poster's description too.

https://www.thespruce.com/damping-off-disease-of-seedlings-1402519https://www.planetnatural.com/pest-problem-solver/plant-disease/damping-off/

I've still got a number of seed trays that I'm trying to nurse along that got it, but looks like a lost cause. I replanted new seeds with far greater precautions this time -- gave a bleach bath to my seed tray 6-packs I'm reusing from last year, as well as all buckets/utentils/water cans, bought new bagged seed start mix (and microwaved it) rather than reusing old potting soil, sprinkled cinnamon on top after planting the new seeds, and sprayed with a neem oil mix to reduce gnats that might carry the fungus spores from other contaminated soil. I'll see how that does. Might be overkill but having lost a month already on my veggie starts, I want to make sure this time.

I also bought a bottle of Garden Phos (phosphorus), which supposedly helps control the most common fungus that create damping off. Only got it yesterday though (to mix a small amount in the water can), so maybe that will help a little too.

I had no idea what damping off was, or how common it is, before this experience.

Thanks Tim and Floral! Yes it sounds like this is what’s happening. I took them off of the heat mat and let the soil dry a little more for the ones still alive. I’ll have to restart quite a few of them. Floral, no signs of snails or slugs, also it’s indoors :)

Damping off was my first thought and is most likely. The Gastropods were just an outside possibility if the seedlings had disappeared completely. In my climate we often get slugs or snails indoors. They come in under doors or get in hidden in anything brought into the house from outside.

When you plant your new batch of seeds, sprinkle a thin layer of dry, finely shredded sphagnum moss on top to help prevent disease. It's called No Dampoff! A tiny bag lasts me years and I do about 10 trays a year.