Phone doesn't turn on, shows apple icon
Got a iPhone 6 and it was working, I just had to reinstall new firmware. The battery went dead and never been able to turn it on since, I've replaced the battery with one I know works as I've swapped them with another iPhone 6 and the battery from this non working one works on there and 3utools shows it's a good battery.
However I when I try and turn it on all I get is the apple logo then it goes off. I've left it to charge and I've plugged it into the usb and done the power button and home button and it still only shows the apple logo.
I need help.