Well, when you were inside it, or who ever before you was and disconnected the power button, could have easily also disconnected, loosened, pinched, damaged, etc., the cable that connects the display. So you may want to start there.
Should that not work/help, and you are certain you connected the external display correctly (used a monitor, turned off the laptop before connecting it, turned on the laptop and made sure there wasn't a button on the keyboard to change monitors) then it could be the graphics driver, the GPU, or the motherboard.
You can try hitting CTRL + SHIFT + WIN + B to try and reset your graphics driver as possible fix.
If none of this works, then you are looking at either the GPU being replaced and/or the motherboard