Both the LCD and battery connectors have a tiny PC board that sits between the MB and the connector. When you disconnect the cable, there is nothing holding them in place and they often come loose and get lost. If the LCD one is missing, nothing shows on the screen. But it will turn on. You can check this by connecting a monitor via the HDMI cable. If the battery one is missing, it won’t charge and won’t turn on unless the power cable is plugged in.