What's happened to my lettuce seedlings??? :-(

Just as I thought I was getting the hang of this gardening thing, I went out to check on them yesterday & alot of my tiny little lettuce seedlings have been uprooted, I've no idea what happened, I'm going to hope some have survived & start a few more on the inside & try & get them past their early, delicate phase before planting out.

Anyone have any ideas what may have went wrong?