Some people will re-freeze, and you can do that --once-- if it's only been out and warmed for an hour or two. Beyond that, toss it. It might be a good idea to ask the breeder which method s/he uses for warming the mice and then replicate that method as closely as possible, if the breeder is reachable. If not, just try to make sure you have a nice, slightly warm, dry, stinky mouse and it might take a feeding or two to get him to eat for you, but he almost definitely will. Just remember that skipping a meal for your snake is like skipping a meal for you, not like skipping that many days' worth of meals.