Not sure how to add to the above. Anyway, it seems to have something to do with confusion as to primary and secondary monitors. If I make the Dell monitor the secondary monitor (via Windows) and restore the HP monitor to primary, the Dell will display an image when I lock the image to the secondary.
So I decided to make the Dell the only monitor and re-boot. Still have the problem.
Should I consider uninstalling lightroom 7.1 and re-installing and re-authorizing? I have my serial number, but I fear that this is some setting somewhere that is sideways, so-to-speak, and the re-install won't effect any settings.
This is clearly a problem between lightroom and Windows, I think. Any ideas? Alternately where else can I go for help?
Since you are on windows, the problem is almost certainly a corrupt monitor profile. Dell monitors are notorious for shipping corrupt profiles in their windows drivers. Lightroom then tends to only show grey squares. You should calibrate your monitor using calibration hardware. If you don't have one of these (a simple Spyder costs $80 and will work well), you shoud open up the monitor's properties pane, go to the color management tab and delete any profile you see associated with the hardware. This should fix the issue temporarily but still won't give you reliable color. For that you need to go out (or go online) and buy a profiler. Owning one of those is essential for digital photography.
Thanks Jao, your hypothesis seems highly likely.
I'll try this tomorrow morning. Tired now (Colorado USA).
MS says it's the standard "plug and play" monitor driver. So I went in via right clicking on the windows home screen and selecting screen resolution. I had no profiles loaded. So I went through the rote MS eyeball screen calibration and just accepted the defaults and, bingo, LR displays images. Will now proceed to fine tuning! Thanks, your provided the key. I am astounded that this didn't work out of the box. I've googled my fingers to the bone and found nothing. Also astounding. Thanks, again. You were a Godsend!
Excellent. Glad it worked. We've seen hundreds of cases like this on this forum now so this is a very common problem with Lightroom on windows using uncalibrated monitors. The built-in windows calibration indeed generates a new monitor profile that Lightroom will use. Surprised you didn't see it in the color management tab in your display properties but it could have been so corrupt that it didn't even show up there. Do note that while better than nothing the built-in calibration doesn't give you very reliable color reproduction. For that you need a hardware calibrator.