As covered in the other answers, your only option is to see a vet in order to have surgery done. Or in the worst case, have it humanely put to rest.
I want to stress how important it is that a professional veterinarian/surgeon takes care of this injury.
A turtle's shell is not only its spine, but its ribs as well. Below is a diagram of how turtles came to develop their shells that we see today. In the diagram of the modern turtle's skeleton, you can see how the ribs are spread out through the carapace (upper half of the shell).
Turtles are slow growers, so unless it's a small crack, it's unlikely that it will heal naturally. A crack as large as yours will have to be rebuilt and sealed in order to prevent water and air from entering the inside of the shell.
As far as what's sticking out of the carapace, I'm not very good with identifying internal organs, but based on its location I would say it's either the liver, or some intestines.
You can reference the diagram below to see for yourself.
Now, as you can see, the crack in your turtle's shell is extremely close to where the lungs should be. You should not try simply press the intestines back inside as you could risk pressing against the lungs, collapsing them, or interrupting the heartbeat, causing a stroke.
I imagine your turtle is pretty stressed out from the injury, so the shock of moving his insides, even if you managed to avoid the lungs and heart, could very well kill him.
If it is the intestines sticking out, you have the risks I've mentioned, but also the risk of placing them back inside in a way that would encourage impaction.
Hopefully you've been able to get him to a vet. Otherwise be sure to keep the wound clean with pure water, keep the turtle out of the tank, but rinse him twice a day to keep him hydrated until the vet appointment.