Just want to ask, how did you came up with replacing your motherboard when you first had the boot problem? What are the troubleshooting steps you did to identify that it's the motherboard that was faulty? Please also do try these troubleshooting steps that may help.
- Start by doing a hard reboot, remove the battery and AC adapter then press and hold the power button for 20 seconds then see if it will boot up.
- Next is to check the ribbon cable that connects the LCD to the motherboard.
- You mentioned that you've already tried connecting an external monitor and it didn't show any display right? This time disconnect the ribbon cable that connects the built in LCD and connect the external monitor and see if you'll get any display and if you do get a display from it the problem lies with the built in display.
- Do also reseat the RAM's, remove them all for couple seconds then put them back in making sure that they are all seated properly.
- Also do try another AC adapter that has the same voltage with yours that may also be the problem.