Here are some things you can do to help:
1. Gently pick up the turtle and place it in a safe container. Make sure the container has air holes and isn’t too tight. A cardboard box or plastic bin with a lid works well.
2. Take a close look at the injury. If the break isn't severe, you may need to simply take steps to keep the turtle clean and dry. Avoid touching the wound directly.
3. If the injury looks serious (like the shell is cracked or if you can see bone), you should take the turtle to a veterinarian or wildlife rehabilitator as soon as possible.
4. Do not attempt to clean or treat the wound yourself. This could further damage the shell and cause infection.
5. Keep the turtle calm and quiet. Turtles can easily go into shock, so it is important to minimize stress as much as possible.
Once you have taken these immediate steps, you should contact a wildlife rehabilitator or veterinarian for further advice.