Probably slugs or snails-they love lupins. Sadly I reckon that with no leaves at all they are ulikely to recover but if you have room to leave them you could a;wys wait and see. Meanwhile if you want to replace them you might surround each new plant with some kind of deterrant. There are lots to choose from nowadays if you don't like the old blue pellets. Or you could make a slug trap - a dish full of beer. They can't resist it, fall in and drown.

As Stera says, slugs and snails love lupins. Having some in a pot, I should have protected them properly but failed to do so. They still have viable flower stems and leaves so now they will be protected.
The pot has a copper band around it but allowing leaves to touch other plants is fatal. Now it's alone and just in case the copper band don't fully work, they are surrounded by sheeps wool. If you plant any more or if yours comes good, I can recommend both the above. Good luck Christenemm.

I never seem to keep more than one or two from a packet of seeds, even with slug pellets!