Tomato seedlings falling over/wilting?

I had some tomato seedlings that were doing really well, about 4-5 inches high and 3-4 sets of leaves. Tonight they have fallen over. Am I giving them too much water or not enough? They are in 4" peat pots and feel dry when I stick my finger in it, but I'm not sure if there is some moisture down further. I'm in Utah so the air is dry here and they were in a south facing basement window, so not too much direct sunlight, but some. Help?