Remove the battery and SIM (if you have on, varies by carrier, if you have Verizon you will not ahvea SIM) from the phone and wait 20 seconds, then, with the battery still out, plug it into your wall charger, then place the battery and SIM back in and turn the phone back on. KEEP it on the charger for at least 30 minutes if it does not start back up and then try again.
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