Which LEDS on MPU are lit during power up etc.
Post pictures of them if you need to
Could be how well the MPU seats into the motherboard try removing and re-inserting MPU into place a few times.
Pressing firmly each time.
Disconnect and reconnect each connector going to motherboard
Lastly you may want to remove motherboard and inspect for damage to the traces etc
One screw will let you remove the metal shield, its at the bottom near the front
Slide cover forward then up towards reels then remove
Then you can remove the 6 or so screws to access both sides of motherboard.
Inspect motherboard for damage