Overnight in the freezer and they could well be still alive next day, just very slow until they warm up.
I used to use a lot of maggots, so here is my advice:
Buy fresh maggots from the tackle shop. The newer, fresher they are the larger the food spot is. Older maggots have almost no food spot, and will soon turn to casters.
When you get back riddle the maggots, take off all the sawdust sold with them. Put them into a maggot tub with fresh clean sawdust or maize meal.
If you buy a pint of maggots, put them in a two pint bait tub. Basically for the amount of maggots you buy, double the size of the bait tub.
Keep your maggots in the fridge, and riddle them every day. This removes dead skins and casters, and check the sawdust/maize meal.
There is a 'but' to fresh maggots, in winter, especially on rivers, older maggots wriggle more on the hook.