Something obviously went wrong. You may need to either send it to service or try Lumia recovery tool to reset software to the factory defaults from your Windows computer. Then try updating it. Hope it'll work this time. I had this with my Lumia 1020. Nobody knows what happened but I managed upgrade after this reset to factory software.
first try to do a hard reset for the phone, if this doesn't work - use Windows Phone Recovery Tool to restore the software, but it will delete all the info from the phone.