Replaced headphone jack; now iPhone won't turn on.

I went through the ifixit procedure to replace the headphone jack ribbon cable assembly. Installed a new battery while I was at it. Everything went perfectly, according to the instructions and photos. I figured I should charge the battery after buttoning it up, so before trying to turn on the phone I plugged in the power. Battery indicator immediately came on in the red zone. Then this was replaced by the Apple logo, so I figured the iPhone was rebooting. Then, the logo gradually faded out to nothing; this took about 30 seconds. I left it charging for about an hour. Now nothing happens, except when I plug the iPhone into power and hit the wake/sleep button, sometimes the Apple logo appears bright for a few seconds, then dims, then gradually fades to black. Once that happens, no matter what buttons I push, including trying a hard reset, nothing happens – screen stays black. Help!