It turns on but doesn't get past the Sprint screen before it shuts it self off and does the same thing over and over again. I actually have to take the battery out in order for it to turn off.
You can reboot the phone using the Android system recovery.
If you are still having issues here are the steps for a factory wipe on the device, this WILL DELETE ALL YOUR DATA on your phone and set it back to a factory state.
My Evo 3D does that. I have to reformat it. Hope you have insurance because it may be a deep problem if reformatting it does not help.
Sprint has tips and resources to help keep them safe and focused.