1st try holding power and volume up until you feel 3 vibrations 1 after the other - Then try to boot your phone - If that still fails then unfortunately you will need to perform a system repair using either PC Companion or SUS found here - You will need to perform the 1st part of this reply then if using PCC click on Support zone then update my phone/tablet then in blue click on repair my phone tablet - Then when prompted hold volume down and connect your usb cable remember your phone should be switched off to perform the repair
pop open the back of the phone , hold dow the off button for 8seconds wait for the device to vibrate 3 times then perform update again via sus (sony update service)downloaded on your pc
