This can happen if power cycled off and on rapidly. I experience it occasionally when I'm too lazy to hold down the designated power button on a case and instead just flip the power supply off and on. I'm not fully aware of the actual cause, but I'm guessing some capacitors were able to hold a charge through the cycle and certain components relying on those capacitors failing as indication that power shut off thus don't tell the graphics cards, mobo, or something else that the power shut off, so it's refusing your command to turn it back on since it thinks it's already on.

I've also occasionally run into an issue where simply turning the power off and on with the PSU doesn't work to "cycle" it and I have to physically unplug it or switch the voltage from 120V to 220V, press the power button, then switch it back to 120V for it to power on. As they say... shit's fuckin' goofy.