How big where they? Just emerged? Did you have them under any kind of plastic or covering or where they very moist?

Dampening off disease can impact indoor plants, they can look withered, or just plain be partly disappeared looking.

That is one of the reasons I gave up on indoor plant starts!

The seedlings were about 3 and had true leaves. They weren't under any covering and were planted in sterile, new potting soil. Not overly moist -- just looks like something nipped the tops off and left the stems in the soil. No sign of the leaves -- they're just gone.

Probably rot! Sterile or not, it happens. It's more the air around them than not. You can try putting a small fan on them, I get clip fans from Big Lots. You can also spray the surface of the soil lightly with $1 store camomille tea. That stops rot.
Sorry about your seedlings, this happens quite a bit. That is why I gave up with indoors (unless in a controlled greenhouse) and use the winter sowing method, but of course it's spring sowing here for veggies. I also use a cold frame. Means they can be late during a year like this year, but they do pretty well.

Fan actually serves dual purpose - the blowing and movement of the stem will actually strengthen the stems of the plant.

For what it's worth, I had squirrels eat the tops off of all of my heirloom tomato seedlings last year, and they all sprouted back, and ended up in several community gardens and neighbor's backyards. If they had true leaves, don't give up yet!

The Op says his/her seedlings are eaten, not wilted, not died, etc.
I have two good sucpects, RATS, and RABBITS.
I have a pretty good experience with rat problem;
They love sprouts, young seedling. So do rabbits.
If, by any chance have noticed any rats around you property,
then you have to work on them.

But your description fit more of the work of rabbits; They eat mostly leaves. But rats nip the stems and eat it.

Well Cyrus, I am giving this individual the benefit of the doubt by his assertion that the seedlings are indoors. I am assuming that rats and rabbits would then, not be a problem.
At least, I hope.

Sorry, GGG
I did not read the post carefully, I guess.

Our greenhouse is having the same problem. Our peppers now have no leaves, the top has a TEENSY black spot on it. The soil was dryish this AM so we do not expect dampening off, (plus we have fans to prevent that, and never have that problem) although we can not see a culprit. To me, the stem doesn't look eaten, so I'm at a loss.

Thankful I'm not experiencing said problems. So far so good :) GGG, I have been seed swapping with a fellow from KY, mostly heirloom tomatoes. If you tend to seed-bank on your tomatoes, let me know, I'm happy to share. Some really fun varieties I haven't found anywhere else yet.

I'm having the same problem as girlgroupgirl but only with a few of the seedlings. Mine are in a storage house under my lights setup, not in a greenhouse.

I agree, they don't look "eaten"; it's almost like something is "capping" the top of the stem. Mine look like teensy brown spots instead of black, though.

Do what I do- let squirrel poop resow your tomato seedlings from last year and then dig them up and plant em where you want em. Easy peasy :) I will never buy or agonize over tomato seedlings again. And every year it's a surprise what I'll get... mine, the neighbors', a delicious mutt of both...

Sattelite Head:
Yep, I seed bank my tomatoes and everything else that doesn't take too long to seed (eventually, I will be banking everything). I would love a trade if you are up for it!!!

Will contact you off the list. I think I have some old emails from you around here somewhere ....

Email you in a few days, or just stop by VG.