snails? I can not grow lupins because they get eaten by snails. tried more then once and give up in the end, I now only grow plants that the snails seem to leave alone, can think of anything else that it could be

Drought, possibly... though in the front garden, if not drought, maybe someone dug the whole thing up and took it away. Planting at this time of year is not recommended, especially during a heatwave with little or no rain, too difficult to keep the new plants alive.

Even so, drought wouldn;t make the flower disappear completely, it would still be there, just brown and dead. If it really was within 2 days and completely gone, that suggests something broke off or ate the flower - but rabbits and deer aren't that keen on lupins.