I would go back and strip it down again to the rebuild slowly and carefully making sure everything is correctly inserted and that the ribbon tabs are locked down. The marked line on any ribbon cables should align with the locking mechanism. This ensures that it is properly inserted into the socket.
If that don't work it may be worth pulling out the CMOS battery ( if removable) to reset the bios settings.
Hope you sort it. I know the feeling when you strip a laptop down only to find it won't turn on properly after