sony xperia z boot loop.

Hi guys..ive been usimg sony xperia z for a while now..i dont use this flashing and rooting things...ive used it completely stock..recently i got an update from sony ..i tried to upgrade it using my mobile data...i had kept for installing and had left it there....after i came back my phone was in a boot loop.it get switched on and displays the sony logo after 2-3seconds shuts down and again restarts and the cycle repeats....I tried powering it up with volume button and power button simultaneosly but didnt work...i also used repair my phone in sony pc companion ..it asked to keep the volume down button pressed and plug it to my pc ..i did the same but it connects and gets disconnected and the phone goes back to loop..i tried it many times but no luck....i enquired sony center, they simply said “u have to change the mother board”.....i never used anu of these flashing stuff so dont know how to use it...if someone can help me get stock sony software by any method please help me...please give a detailed method also if u can send a video ,id be thankfull...thank you,,hope my phone is repairable please please please help....thanks again....writting this after reading many forums blogs...