Sometimes this doesn’t happen quite immediately, but the result is always the same – Windows thinks there is no monitor connected, thus the desktop is not rendered and remote control of the computer (via VNC in my case) is not possible or suffers strange graphical glitches.
In the case of my 4K monitors, as they are connected by DisplayPort, powering them off via the front panel controls results in them “disconnecting” from the graphics card.
Having recently upgraded to a GTX 1070 Ti graphics card to keep up with my dual 4K monitor configuration, I came across an issue that plagues many Nvidia chipset users – namely that of what happens when you power off DisplayPort connected monitors.