Thanks for writing! It sounds like your seedlings suffered from damping off --- a very common fungal disease. When damping off strikes, part of the seedling's stem rots (it's usually the point where the stem meets the ground). When it rots, the seedling tips over like a felled tree.
Prevent damping off by encouraging good air flow (some gardeners set up a small desk fan to blow on their seedlings as they emerge to ward off damping off). Also: Make sure you're not watering too much --- once seedlings sprout, you can let the seed-starting mix start to dry out a tiny bit between waterings.
---Justin, Senior Garden Editor, BHG.com "