iPhone update - unknown error occurred (53).

IPHONE 6

Yesterday I connected my iPhone 6 to iMac Version 10.11.6 (OS X EL CAPITAN) to charge it

up. I got a message that there was a software update for the phone and I selected the

update.  I then got this message: iPhone could not be restored. An unknown error occurred (53).

I tried to restore factory settings and again got error message.

My phone is not usable now, the screen shows the iTunes icon, with message

supportapple.com/phone/restore

What can I do to get my phone working again?