Really sorry to hear about your little ones.
This sounds like what's called "damping off" this typically occurs when the seedlings are planted in cold, wet soil and is further increased by poor soil drainage.
The biggest beginner error is overwatering, this is easiest to do when the plants are small.
Make sure you add lots of drainage to your soil (2/3 compost 1/3 perlite) and that the pots have adequate drainage and dampen the soil but don't soak it and leave it to dry out between waterings
Hopefully if you stop watering now and make sure they don't get cold the others will be okay and you can chalk it up to experience
Good luck I hope it turns around fo you, please let us know how you get on.
That's how we learn best,by or mistakes. You probly wont ever do it again. I did same. The 1's that lived i almost killed another way. Over feeding. I learned less is more. Than later learned how to adjust feed. Till the tips only burn. Took a long while to adjust. Start weak, add lil bit more untill the tips start burning( no more than 1/8") . my soil mix is like , the water/feed almost pours out bottom as i'm pouring in. Use rice for water retention. Towards end of grow the rice has composted and used as food.✌