Black screen of death

I’ve been working with a Microsoft surface Pro 4 tablet, and have plugged in the screen to the logic board but have **NOT** seen the screen come on after replacing the battery, however the LED on the charger illuminates when the charger is plugged.

I tried resetting the tablet using the;

• hold power button for 30 seconds, release button

• then hold power and volume up button for 15 seconds, release buttons

• then wait 10 seconds and press power button

I tried that procedure with the table plugged to the charger and not plugged in, but still did not see any activity from the screen.

What would be the next logical thing to check and see where the fault would be that is preventing this tablet for booting?