As long as the turtle skin is not penetrated, you can repair the shell with a polymer epoxy resins and fiberglass, painting in on in layer. If this is an adult turtle, it can be permanent, but if a young growing turtle, you will need to replace it as he grows usually or patch it with the same product.
Here is one person's guide on replacing a damaged shell that can help guide you. I do recommend you rough up each layer with sandpaper, before applying the next on so the replaced shell is protective and not easy to break open. http://www.anapsid.org/shellrepair.html You basically are going to cover it like you would a table or wood chest and products can be found in stores like Home Depot, other building supply stores or craft stores, Please let me know if you need any additional help or questions on this process.