How likely repairable is a dead iPhone 6 +?

My iPhone 6+ died a couple of days ago. I had not yet upgraded to iOS 9.3 so I was still running 9.2.1. It seemed to do a spontaneous reset and then spent a long time on the apple logo boot screen. I went off doing other things assuming that it would come back like it normally does when it decides to reset itself, it seems to do that periodically.

Now there is no response. I've tried all the known reset methods, held the buttons for way over 10 seconds, etc. Left it on charge overnight. Nothing.

The phone has never been submerged in water, and no physical damage on the outside. It has been dropped multiple times inside a bumper case and has worked after each time. Any water exposure is from weather or wet hands. It was a refurbished phone purchased from ATT about a year and a half ago. No insurance, or protection plan.

I'm about to take it to the Apple store nearby and see if they can do anything to resurrect it. If not, then I'm wondering about repair probability. I would likely take it to a shop, unless it's a really easy thing to try out. I could do the repair work myself, I don't know if I want to take the time.

Any thoughts? Loose cables from dropping? How often does a phone just go dead, especially after appearing to reboot? Is there something else I can do that the Apple Store Genius might not be willing to do?

Thanks,

S