Was the phone dropped or came into contact with water in the last few days before the failure?
Electronic devices can fail spontaneously. There are thousands of individual components in the device, pretty much any one of them failing can cause the phone to fail as well. If you have a copy of your data, just go to Apple and have the device replaced as it is still under warranty. If you need to recover the data, then find a reputable shop that does micro-soldering repairs.