Has it been dropped or wet? Have you installed any new app or updated anything before this?
Try pulling the battery, putting it back in, leaving it off. Then press and hold the volume+, home, and power button until the phone boots into recovery and do a factory reset ****CAUTION. ... YOU WILL LOSE ALL DATA THAT HAS NOT BEEN BACKED UP VIA SD CARD OR SYNCED TO YOUR SAMSUNG/GOOGLE ACCOUNT ****
hope that helps
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