Sounds like damping off.
Have you tried Winter Sowing your seeds? There is no problem like that using this method. It's easy. It's fun and gives great results.

Here is a link that might be useful: Winter Sowing FAQ

I am a sometime visitor to this forum and an infrequent poster but this question beckons me.
It could be damping off, which is caused by fungi.
Contaminated seedling medium is often the cause. Conditions that are cool and damp exacerbate the problem. Seedlings appear to just topple over. Make sure your containers and flats have proper drainage. Avoid planting too closely and keep the planting medium evenly moist but not wet. Even if you are using a sterile medium, used pots, flats and planting tools can harbor contamination. You can wash them using very hot water (160 degrees), or make a bath using a 10 to 1 solution of water to household bleach. I had a damping off problem in the greenhouse one year which I attribute to used flats I purchased from another grower which I foolishly used without cleaning.

There are other possibilities. Overwatering can cause plant death without the presence of disease organisms and is another common cause of death for not only seedlings but established plants in containers. After germination, most seedlings should be kept only moist enough to prevent wilting. Conversely, allowing them to dry out completely can also destroy burgeoning plant tissues. Extremes of temperatures can cause problems, too.

Good luck!

I agree with damping off. When I winter sow that's never a problem. Unfortunately not everything can be WS'd. Poisondartfrog gave you some good info. I've also tried misting with diluted hydrogen peroxide which seems to help. Fungus gnats can also be a problem. Watering with Bt seems to keep them in check. I drop a Mosquito Dunk in a jug of water and use that on the seedlings. Good air circulation is also necessary. You can set up a small fan near your trays. Good luck, hope the rest of your seedlings do well.
Karyn

Thank you all. Your input is all important to know and learn. I appreciate it very much.