Well that's not good but have tried plugging it into a USB cable yet? When I tried W10 on my phone it 'died' on me but then came on when I plugged it in to USB.
Also holding down the power button will sometimes cause a 'reset' to perform and it may turn it on.
That is how it was. I downloaded this How to roll back from Windows 10 Preview to Windows Phone 8.1 | Windows Central and started the process. It showed a progress bar on my phone. Dont remember if it was any movement on it. Then the software said that the process has failed. Restart the program and connect your phone or something like this. After that the phone is not powering up.