Probably slugs or snails or both - slug pellets!

Thanks Bamboo.

Should I leave the plant in..will it grow ?

The plants will only survive if the growing point is still ok... the bud right in the middle of the plant where it's producing the bean 'vine'. New leaves and a shoot will come from this.
If the growing point is damaged, I'm afraid they will be kaput.
If I were you I'd just sow another dozen or two in a module and put them somewhere warm like a greenhouse or sunny windowsill. They will be up in a week and you'd be able to transplant them in 10 days.
Healthy new plants will probably outgrow the damaged ones, even if they weren't killed, and in the end you will probably find you are cropping them as early as anybody else's.
(I've just done exactly that with some Spanish kidney beans as the first ones sown outside all succumbed to snails or maybe the goose!)

Many thanks Bertiefox.

if there are side axil buds it will sprout from the sies. Mine have done so after snails got to them first.

Thanks Seaburngirl...will take a look.