My iphone 6 went black suddenly! (URGENT)

Dear all,

My iphone suddenly went dead this morning. While it was on my hand, it was not wet, no impact at that moment, and still 85% full of battery. After It went dead, it cant be on, have went to local iphone both official and private shops and seek for help.

Tried new battery but the iphone 6 couldn't be on at all. According to the specialist, the phone crashed and it is almost impossible to retrieve the data. However, some experts advised me to send the phone to china to retrieve back up. before letting the phone lying the garbage forever.

With the phone unable to boot at all, it is almost impossible for me to back up on itune before transferring all my data to a new phone.

May any kind soul please advise?