This sounds like a classic case of damping off - a fungal infection that quickly kills new seedlings. It is encouraged by poor air circulation and damp soil. Be sure to use a seeding mix that drains quickly and a fan near the seedlings may help. Do not let them sit in a cold spot such as near a glass window at night. Before you start over, wash any pots or tools in very hot water so you don't spread the disease again. Now don't worry - seeds are cheap and this disease does not return once you learn how to prevent it. Keep growing, Marianne Binetti