Sounds like damping off disease - this is a fungal infection and is a real pain once you've got it. You will need to start over with your seedlings, but either buy new pots or sterilise the pots and trays you already have, use new seed and cutting compost, increase air flow and light levels around the seedlings, don't cover them, don't overwater, allow compost surface to dry out slightly between waterings, never leave trays or pots standing in an outer tray full of water. Occurs more frequently on seeds started indoors or in a greenhouse, but can occur outdoors too.

Thankyou

Next time, water the newly emerged seedlings with cheshunt compound with a very fine rose watering can. Doing it right away with your present seedlings may save the rest of the tray too.