After I changed to a newer video card, shut down and started again my monitor has been dark, no display. RAM, processor, drive are all OK; the video card is in its slot and I have confirmed that all cards are firmly in their slots.
Does anybody have an idea of what I should try next?

It might be that the motherboard doesn't recognize your card. Try using another card to change bios settings and then it should work

If that doesn't work use another card to install the drivers

its impossible to say whats wrong without knowing some more info about you computer ,like the motherboard make and model #.

its impossible to say whats wrong without knowing some more info about you computer ,like the motherboard make and model #.

caperjack I think you are right

Did you go from an onboard video to an actual card? You might need to boot to safe mode to get the standard VGA drivers to come up, then reinstall the drivers for the new card with the standard VGA drivers in place.