If it's one of the bungs with a hole in it, get a metal crochet hook - size K works best. It's the largest they make that's still metal. You can buy one at Walmart for less than $2. - Grab the stopper through the airlock hole. (You may need to turn the carboy to do this.) You'll need to wiggle and turn the stopper as you pull. Don't pull from one spot on the stopper or it will start to rip. Pull a little here and a little from there. It helps if the neck is wet. Once it's just over 1/2 way in the neck, it gets really easy to pull the stopper right out!
I've become very proficient at this technique. And yes, now that I bent my crochet hook, I actually have one dedicated to meadmaking.