The idea behind growing tender plants as die-back perennials is that the earth is a good insulator and many times a plant's rootstock will survive moderately cold weather that kills the top growth. If the plant is well established in the ground before the freeze, and well mulched, dormant buds near soil level may survive to shoot again the following spring.

Keeping plants in containers changes all that as the roots are above grade and exposed to the cold. The best thing at this point would be to put them in a cool greenhouse. But honestly, they are so easily planted anew as seed-grown annuals that I don't think I would bother. You could also put them in a garage or other freeze-sheltered spot and plant them out if they show signs of recovery in the spring. Next time, if you want to try growing them as perennials, get them in the ground in the spring.