Accidentally tried to set time to 1969 on iPhone 6 and crashed
You can't restore out of this problem, even with DFU restore. A bug with 64-bit devices renders them ‘bricked’ if you set the date to 1970-1-1.

Take your device to an Apple Store for them to reseat the battery, or you can try reseating the battery yourself. You could also put it in a drawer and eventually the battery will drain to a point where the date setting is lost and it should then power up properly. At this point, don't charge it and seek hardware service if you can't wait out this situation.

The bug is discussed on /r/apple: Changing Time &amp; Date settings to Jan 1, 1970 will permanently brick 64-bit iOS devices