Pythium fungus, as well as other fungi, causes the seedling leaves to wilt, whether the soil is moist or not. The stem may look like it's bent or even be creased. There is no way to save the seedling. Use sterile potting mix when planting seeds and rotate the crops in the garden.
Seedlings grow quickly. If they're water stressed they may not recover because they have few energy resources until they've grown bigger. If the leaves are wilted, water the plant. If the entire plant is wilted, it's not likely to recover.
The fungus that causes the stem to rot and leaves to wilt in green bean seedlings builds up in the garden soil over time when beans are planted year after year. Rotate the crops to a different spot each year.