Leave it on till it completely dies, then keep turning it on until it will no longer stay on. Wait about 15min then charge it full and turn it back on. If that doesn't work, try a Hard Reset? If that doesn't work, I would take it into the Sprint Repair center as something else is terribly wrong.
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Keep in mind, like you, I am a Sprint subscriber. I am in no way employed by Sprint in any sense. I just know stuff.
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