I've done the following:
-- ran diagnostics on monitor (it says it's OK);
-- made sure cables were snug;
-- downloaded and installed latest driver;
-- switched HDMI cable.
So far, still no HDMI image. I wonder if the sole HDMI port on the m'board might have failed (device Manager doesn't show any problems)? If I had another HDMI monitor I would try that, but that's not an option. I could pull a TV from another room to try, but they're all so big that would be a hassle. But is there any other way to check the signal?