What happened?

Last night we had a pretty bad storm here and the kids and I were out.  I had thought that I put the chicks back up before I left, but when we returned home I found 5 little chicks huddled under the coop and the 6th is nowhere to be found.  I was hoping somehow she would be in the coop and waiting to rejoin her friends when I woke this am, but I still see no trace of her.

Since I don't know what happened, I think they will not be having anymore outings for a couple of weeks w/o supervision.