if they are completely stiff and frozen...don't bother... even if you do get them started again they are delayed and easily die again.
My protocol depends on how many litters I have and how old they are.
Ideal situation...would have a litter one week older, then I just plop the chilled kits (so not the unmoving kits...just the quite cold but still moving ones) smack dab in the middle of the group. They warm up SO fast it's amazing.
COLD, not moving. I use my cleavage to good advantage.
Once they are squirming I plop them into a warm litter and get them warmed through the rest of the way.
In a less than ideal situation.
I do chores with cold kit tucked into my arm pit (or use my cleavage). that keeps them warm enough until I'm done chores and get can back into the house.
Then I do a couple of things. Using a bag and dish warm water, get them warmed up and moving. Then I wrap a warm/hot towel around the kit. My goal is to get them warmish and moving.
Once they are warmish and moving I pack them up in nest with straw, with a small cubby made for them with TONS of hair. I keep all my clean dry hair, whether it's been mixed with straw or not.
That will save them.
Litters with three or less come into the house at night for one week.