The phone definitely is charging and previously set alarms go off on schedule, but screen stays black.

Is there a way to check the integrity of the ribbon cables?

Excellent guide.

After reassembling to step 17( reconnecting the battery) I tried powering on the phone. Nothing happened. I connected to a charger,, the phone vibrated and the red blinking light appeared. I left the phone connected to the charger for an hour, tried to power on and nothing happened. I checked to make sure all the connections were tight, but to no avail. I tried a soft reset, but that didn’t work either. Any idea what I did wrong?