The unit would nothing no lights no controls

I checked the incoming power and main fuse. Both are good.removed the microwave from the wall mount, removed the door and outer cover. Removed the right side panel and replaced the main PCB. I found loose mounting screws for the vent fan and other placecs so I tighten all the hardware I could see. Pluged the unit and found the display said input time and date and the lights turned on when the door was opened and the vent worked. installed all covers and checked the microwave operations.