This is probably too late for you now, you’ve probably solved this. But for anyone else experiencing this issue, it COULD be the power options. Windows automatically changes the screen brightness when you unplug, you could’ve accidentally set it right down. Go to Control Panel (Small Icons view) > Power Options. There should be options in there for what to do when removing the power adapter. Sorry I can’t be more specific, I’m on a desktop so it doesn’t have the right options to explain it fully.