What do you see, when you dig up the area? For just a few plants, I would suspect gophers or moles.

How is the drainage in that bed? Have you previously had successful plantings in same area?

Without knowing just why the plants died, would be hard to know what preventive measures would be needed.

This is a common problem here in the south. Can't tell you what causes it; maybe voles/moles. But I do know how to stop it. I keep an old plastic watering can just for this purpose which I fill to the top with water and then stir 3 tablespoons of castor oil using a long stick because this is a gooey mess. Plant the plants and water them in with the contents of the watering can. Repeat watering once more a week later. Tough to convince gardeners to use this method, but when they do they are pleased with the results.

Voles, not moles, will eat plant roots so if the plants died overnight most likely it was voles. Any soil drench that might be effective against soil dwelling insects will also kill off the Soil Food Web necessary to grow the plants you have. Insects will not devour plant roots that quickly, anyway.

If these are young plants the problem might be cutworms. Using any kind of systemic poison without identifying the problem is not compatible with good organic practice, and likely a waste of time and money.