After repair: One only shows battery bar, but doesn't start!

Hey ifixit guys,

I got a HTC One M8 for repairing the Display. I also changed the battery because I broke it. After reassembling, the One seems to load the new battery. It shows the loading screen, but the orange LED is blinking all the time for unknown reasons.

Also pushing the power button only brings the phone to vibration, but android/fastboot or anything will not start.

Could anybody please help me?