Same problem happened to me after replacing the RAM and sealing the display just to find that the display was not turning on. I eventually fixed it but to be honest, I don’t know what was the cause for it. I did my upgrade back in April 2020.
I disassembled it again with the purpose of making sure I properly re-attached every connection and/or making sure I didn’t missed a step. It is very possible that one of the many cables that was disconnected was not properly reconnected (pushed all the way in). You don’t need to reassemble the display to test if everything is properly connected since there are three or four diagnostics LEDS that will progressively turn on when powering the unit. The last LED is for the display test, which will fail (not illuminate) if the display is not connected. If all LEDS turns on except the display, then you know that everything is working properly except the display (which is not connected). Then you can proceed connecting the display to verify its functionality.
You may Google regarding about the diagnostics LEDs for more information since I can’t recall what each LED display represented.
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