Have you loaded up your mayor or any other existing character on the day they're in boxes? If so, there's nothing you can do anymore as far as I know. If not, they can still be saved, but the method I know of doesn't involve making a new character.
Well if you know they are going to be in boxes on whatever day it is. dont load your mayor or it wont work. you open a new save file find the person. then you use the system settings to tt back a couple of days and try to get said villager to ping so you can prevent them from moving