Whatever is wrong with your turtle, it's time to see a vet.
If the shell is cracked, the vet is the best person to help you decide what is best to do. There may be a wound that needs to be cleaned, and there may be repairs to make.
If the plastron (bottom shell) is red, your turtle probably has a blood infection, and for that, you really need a vet.
Either way, it's better to be safe than sorry.
It would be great if you could post back here to tell us what happened.