DIY Urban Gardening - Lettuce Pot Sprouts not doing well

I don't know what happened to this lettuce pot that we had going. It was looking really good when we took it from inside and put it onto the lanai. It was doing well for two days but when I came home yesterday they were all lying down. It wasn't even a sunny day yesterday.

I really wish I knew what causes this as this is only the only pot that is not doing well.