What's happened to my brassicas?

All my beautiful little brassica seedlings that I have been nurturing in my (plastic) greenhouse until the weather gets better, have all apparently been ravaged by something and I'm not sure if this means the end of them.  I carefully potted them all on at the 4 leaf stage and, because it's been so cold and wet, kept them all inside but when I went round at the weekend all there was left of them was a stalk in each pot - they looked like they'd been eaten by slugs but there wasn't a slug in sight.  My toms all seem to have escaped any damage.  Any suggestions to what this might be?  And will they recover from this or should I start again?  Thx