Pixel won't fully start after battery swap.

I replaced the battery and screen of my Pixel following all directions on ifixit's website. When I turn on the pixel, the Google start screen comes on for about 5-10 seconds and then the screen goes blank for 5-10. Then it repeats. On and off, plugged in or not. The phone never boots up, just cycles endlessly. Any idea what I could have done wrong or what I can do to fix this? I've not yet glued the screen back down fortunately so I took the mid frame back off and checked that all connections were secure, but this made no difference.