Honestly, this sounds like the AMD "monitor wake / black screen bug". It seems like it needs the monitor to fully reset the HDMI hand-shake (by unplugging it), and plugging it back in, to trigger a re-detect.
Assuming that you're using HDMI, try moving the mouse, unplugging the cable from the PC and plugging it back in, does it come back then? If so, that's likely to be a video card / driver problem.
Didn't you have some video card issues before? Cards not installing the drivers correctly, or detecting correctly, or something?
Maybe you're right. The problem got worse. I don't recall the exact steps, but basically it wouldn't restart at all, even when I'd take the power cable on and off. Restarting the PC didn't help either.
It was sort of showing it was connected - when I stopped the screensaver, it showed the screen contents a second, and then went black. But then it would go to 'no video signal' and stay there.
Not everyone has all this crap, but I had a second PC hooked up to it by VGA; I had a spare HDMI cable; and I even have a new monitor, a 34" I bought 18 months ago and never hooked up. Let's get started.
Unplugging the second PC, no effect, and it wouldn't work on the second PC either. Really looking like a bad monitor. Switched out the HDMI cable just in case - nothing.
So, finally, I unpacked the new monitor and hooked it up - other than standing on a 6" elevated base so it's too high, working like a charm. So I conclude unless it was that bug you mentioned and unplugging the HDMI cable would have reset it, that the monitor is bad. Time for this new, really bright, 34". Man it's wide.