Is the screen completely black? Do you have xscreensaver configured to be black-only or is it running savers? If you are not running savers, try doing that, to see if, when it is in this "invisible" mode, there are any pixels visible at all.
My guess here is that a bug in your video driver is causing the screen to either not be powered on, or is causing the frame buffer to not be displayed. That is, the client-facing side of X11 believes there are bits on your screen, but the hardware-facing side has lost its mind. -- Jamie Zawinski https://www.jwz.org/ https://www.dnalounge.com/
