Mac screen goes black instantly when power cord is plugged in

My Mac laptop is half charged... and the second I plug in my power cord, the screen goes black. Pushing any keys on the keyboard does nothing. Then the second I unplug the power cord, the screen comes back on. I thought maybe it was trying to do an update, so I cancelled the upcoming update.... but it still did the same thing again when I plugged in the power cord. HELP!