When you overtighted that the pins and cpu socket holes most likely saw a slight gap from spreading a little. Then there was no continuity stalling the system there. You may have seen the cpu lift up slightly on one side or corner for that or simply made a contact on the board loose like where those pin sockets are soldered to the board.
The one lesson I think you get from this is never to slam anything down hard when putting a build together.
It saves a lot of headaches from being seen.