Iphone 6plus doesn't start after reset. Just the Apple on the screen. what can i do?

My iphone had this problem. I left if overnight and it seemed to resolve, so today it seemed to be working fine....

Then I added passcode and touch print... then it has done the cycling between slide to upgrade message, at least then it responds to sliding which it did not yesterday... then wants ID, then it goes to download screen - apple symbol and line.. after grey bit grows to 3mm it locks and turns itself off.

This really is apalling......

Phil