HT201559: Restart your iPhone, iPad, or iPod touch
Learn about Restart your iPhone, iPad, or iPod touch
Q: iPhone 5S 8.2.1 won't turn on, hard reset, or restore
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Jan 26, 2015 6:11 AMby Matt_M.,Hello Kwon Jiyong,
Wow! That definitely sounds frustrating. In these situations, the best possible step would be to isolate the issue to hardware or software. To do so, you’ll want to restore your device with iTunes.
Use iTunes to restore your iOS device to factory settings - Apple Support
http://support.apple.com/en-us/HT201252
However, it sounds like you may have already taken this step. If not, definitely do so by following the resource above. If so, then it sounds like your iPhone may need to be serviced. The following link should help you get started with the process. Included, you will find information on topics such as warranty & service pricing.
iPhone Repair and Service - Apple Support
http://www.apple.com/support/iphone/repair/
- Matt M.
