The Sky is Falling, by Lester del Rey

I'm not usually fond of fantasy but this is a clever little story. In an alternate Earth, the universe is an egg and the sky is cracking. In a desperate attempt to save their world, the local magicians snatch someone from our Earth: the greatest builder in history, Dave Hanson. Unfortunately there has been a miscalculation. Instead of Dave Hanson, genious engineer and renowned builder, they've snatched his nephew: Dave Hanson, computer repairman.

But because that's how prophecies work, our Dave Hanson is the right guy after all.