...So, summing up:
I did all of the window-cleaning advice given in the often-quoted HP troubleshooting article;
I did the hard-restart several times, to no effect;
I uninstalled/reinstalled the latest software (which ws already installed);
I updated the firmware (which was already up-t0-date);
I updated the driver (again, this had already been done);
and then called HP support.
At that point, they asked me to re-do all of the things I had already done. After they didn't work (again), they had me do a "Semi-restore," which didn't work, but also had the effect of wiping out my Wi-fi connection settings.
Then, they had me re-connect my Wi-Fi settings for a final test...which didn't work.
At that point, the problem was diagnosed as a "hardware failure," and they are currently sending a refurbed unit to replace mine.
And it only took an hour on the phone.
Good luck!