I had the very same problem with my previous 520, apparently it was some hardware related problem. I had to send it to Nokia Care and they replaced my device (took 18 days for some ridiculous reason). I'd recommend you to try Nokia Software Recovery tool first, although I doubt that it'll do any good.
? Download and install Nokia Software Recovery tool from MS site.
? Connect your phone with your PC through the USB cable that came with the phone (it should automatically detect the phone, if it doesn't try another USB or change USB ports).
? NSR will guide you through the whole process, and wait for it to re-flash your device.