Andrew Braverman, You did the right first step, by opening up and checking connections.
Next, use a USB meter and connect between the power source and the USB charge cable, and check what power is being drawn (amperage) when just connected, and then when initiating boot with the power button. Normal current draw when charging is ~0.95A.
One test you can also do, is to plug the phone into a computer and not a charge brick. If the phone is “alive”, you’ll hear a “ding” from the computer. This indicates that the phone is being registered and you most likely have a bad screen connecting and/or bad screen.
If the screen is the issue, open it once again and unplug battery then screen. Clean all connectors with >99% isopropyl alcohol and dry them off for a minute. Reseat the screen and then battery before testing it again.