First, thing I would do is charge it up to about 20% or so and then plug it into your computer and try to get it into DFU or recovery mode. If it is unable to do that, I would hook up a USB digital ammeter to your power brick and cable. This will tell you how much current is being drawn to the device. If it is under .90Amps I would replace the battery. If it doesn’t fully boot after the battery replacement you are likely looking at a logic board issue. Hope this helps!