After replacing digitizer, why does the screen power on and off?

I replaced a cracked digitizer, I used Kapton tape as instructed, and now I'm testing it to make sure it works before I stick it together. When I first pressed the power button it booted up with the white Apple against the black screen. Then it went to the wallpaper screen. Then it went black like it powered off.

I plugged it in, waited a couple minutes, then turned it on. It made it to the white Apple against the black screen, then it shut off again. If I unplug it, it won't power on at all. If I plug it in it will turn on just to the white Apple/black screen, but it won't get to the wallpaper screen again. It will turn on and off.

I tried resetting it by pressing the home button and power button at the same time, but it does nothing.

What do I need to check, or what did I miss or do wrong, or is this a sign of a faulty something or other?

Thank you for your help!