Damping Off’ is a soil dwelling fungal disease that occurs in cool wet, poorly ventilated conditions. It can prevent seeds from germinating and cause seedlings to collapse & topple over as the base of the stem withers. Sometimes a fine cotton wool thread like appearance can be seen on the surface of the compost/soil.
To help prevent it occurring, use quality sterile seed sowing medium (compost/soil), use clean pots/trays, ensure the medium is not waterlogged or overwatered, sow seeds thinly to avoid overcrowded seedlings and have good ventilation as this reduces humidity which the disease loves.
If ‘damping off’ occurs remove infected seedlings immediately and you might just be able to salvage a few healthier seedlings by referring to the tips above.