The same thing happened to me last year. I had about 50 very healthy and all disappeared in a matter of few days. I decided never to grow them again. I remember there was a similar question last year and someone answered it. I do not remember who but I am sure you will receive an answer soon.

I planted 2 this year side by side in pots one has flourished and the other was eaten overnight I presume by slugs?

Apart from Greenfly, keeping Busy Lizzies in pots works for me.

I have a very sluggy garden but find that the Busy Lizzies are mostly OK apart from a few flowers nibbled.I'd look for another culprit - pigeons? rabbits? human thieves? I've had plants stolen out of the garden before.