Your seedling stems looked about like this?
pinched in and a bit brown at the bottom, just above or just below soil level?
If so, that is a fungal disease called damping off. It cannot be cured. Once they have obvious damping off signs, they are goners. Sorry.
But it can be prevented. Seedlings are very vulnerable to damping off in conditions of too much moisture and too little air circulation. Are your seedlings indoors? If so is your weather such that you can start putting them out, at least in the day time? If not, you might invest in a small personal fan to blow over them, for the air circulation part. Only
water your seedlings from the bottom.
It helps to put a little bit of cinnamon in the water you water with-- cinnamon is a natural anti-fungal.