I started some seeds in my garage a couple weeks ago. The seeds germinated well, but when the were 2-3 inches tall they fell over and died. This also happened to me last year. The plants were green peppers, brocolli, and cabbage.
Could be to cold,lack of sunshine or not enough water. Just a guess.
And also the soil might not have the nutritents the plants need.I always buy the potting soil made by miracal grow. I get it at Lowes.
Damping of is a fungus that "cuts" the new shoot of a plant when it is in its succulant stage. Plant just falls over. Most of the time it is caused from giving the seedlings too much water or having them in too humid of conditions. If using a cold frame or one of those plastic covers you should open the top to allow air flow once the seedling sprout. Also need to lay off the water some and let the planting medium dry out some. I always take mine out and put them in a semi shady spot like my porch where they can get stressed by the elements some. This will harden up the sprouts some and make them begin putting on true leaves. Once this happens the chance of damping off diminishes.
Good answer by Jogee. The important things to try and do are: (1) Start with a new bag of growing medium(it will be sterile). (2) Give plenty of light(preferably Sunlight). (3) Allow (or manipulate by draft or fan) air circulation. (4) Keep temperature close to optimum(between 60F low and 90F high) This is the hardest step to accomplish. Don't sweat it if it goes a little over or under. (5)Don't overwater. Try to keep as dry as possible without drought stressing. Water early in the day so the leaves can dry off before the dark cold night.
As Jogee said, damping off is a fungal disease and will be hampered by following the steps I listed.
One other step I thought of. If you're using old containers(and who isn't) you should wash them in a 10% bleach solution. It will kill any residual fungal spores.