you probably have "damping off" disease, southern blight, or another fungal disease.
From: http://www.clemson.edu/extension/hgic/p ... c2217.html
Prevention & Treatment: Damping-off is often a problem in plants that are planted too early in the spring. The fungi are more active in cool, wet, rich soils. To prevent damping-off, take these precautions:
Start seeds indoors in sterilized potting mix.
Do not start seeds in soil that has a high nitrogen level. Add Nitrogen fertilizer after the seedlings have produced their first true leaves.
Allow the surface of the soil to dry between waterings.