It's probably damping off, and usually results for dirt that is too wet and/or insufficient air motion. Did you cover the seeds? If so, how long did you leave them covered?

In general, I try to only cover them for a day or two unless I know they are very slow to germinate.

Cinnamon will help with the fungus if you put it on the fuzzy stuff, as will sun and air movement. For those seedlings which are not dead, that might save them. Worth a try!

I have a lot of trouble avoiding this problem with my own seeds, although I do find that I'm getting better as I learn. My main problem is that I tend to get the soil too wet, then cover it which makes it hard to get less wet.

Here is a link that might be useful: Damping off

Thanks for the damping off link - you seem to have nailed it. I planted Saturday, and they're still covered. I'll remove the covers as soon as I get home!

You're welcome, and good luck!

Soil for planting seeds should not be so wet that you can squeeze water out of it, FWIW. And yes, don't keep them covered too long. :)

I do a lot of winter-sowing and start most seeds outdoors now, but I started seeds under lights for nearly 2 decades. Growing seedlings indoor is a tricky balance of keeping seeds and small seedlings sufficiently moist, but not too wet which can encourage mold and fungus. A cover is okay, as long as it's not too humid underneath, and I remove those once the seedlings got their first set of leaves. I use one of those pump hand sprayers to moisten the soil for seeds and cuttings which makes it harder to overwater.

Be sure to use clean seed containers and a sterile soil-less potting mix. I never used a heating mat, because I usually found that the heat generated by the fluorescent fixtures was sufficient. A dilute liquid fertilizer every couple weeks is good when the seedlings get a couple sets of leaves. Sometimes I would use a dilute organic copper fungicide to prevent damping off, but only occasionally had that problem.

Cinnamon works, as does the spray antifungal you can get from Home Depot for $5.00. It isn't always fatal.