Why can't I start up after top case replacement?

I replaced my top case and it seemed to go smoothly. The only issue was that I may have initially reconnected the keyboard ribbon incorrectly by sliding it under the connector rather than into it.

Upon my first attempt to power up, I got to the login screen with the keyboard back light on but the keyboard was unresponsive so I could not log in (the trackpad did work, though).

I examined the keyboard ribbon connection and reconnected it in the way that seemed correct this time. When powering on this time, the process got stuck at the gray, spinning wheel screen. I've tried several times with the same results. Disconnecting the keyboard ribbon altogether and powering up that way still produces the same gray screen result.

Any ideas?