Hey Dan. The first thing to try, now that you have the device open, is a known-good battery. If the device is old and it is the original battery, that would be a likely culprit. If a good battery doesn’t solve the problem, then you may have a charge port issue.
Of course, once those things are eliminated, you are left with a logic board issue. DO you happen to know the history of the device, such as if the device was dropped, came into contact with water ar was repaired by another sop just prior to the issue? That may give you another clue to help with the troubleshooting. Otherwise, you will need to probe the logic board and check if the main voltages or PMIC outputs are shorted to ground.