And it won't turn on under a certain charge level. So let it get to 50%
Then press and hold the power button a little longer. Mine isn't instant on when the battery ran out. I have to charge then turn it on with a longer press.
I left my Note5 on the table overnight and i suddenly wont boot. I can tap the home button when its on charge and can see how much it has charged, but it wont boot up.