Resolution on a laptop was accidentally set to a really low value. When the laptop was woken up from sleep mode, screen was completely black. I realized it wasn't displaying because it couldn't display when the resolution is set too low. So hooked up an external monitor and there was my desktop screen. The problem is that for some reason it won't let me switch my laptop screen back to its designation as the default monitor. It only outputs the data to the external display. In the Display control panel, the external display is designated as monitor #1, labeled as Generic PnP monitor. the laptop screen is monitor #2, default monitor(which is completely black, receiving no signal). It won't let me increase the resolution on monitor #2 so that the laptop screen can receive a signal again.