Boris wrote:

I have lots of LCDs now.

Does the monitor have input select? That is, does the monitor have VGA,
DVI, and HDMI inputs? If so, have you tried using the monitor's control
to switch between inputs to see if it detects an active signal on one of
them?

Did you do any customization in the LCD you are using now? If so, does
it have a reset (back to factory-time defaults)?

Can you test with a different LCD monitor on the problematic computer?