Electronics Fault Diagnosis

Could you clarify the problem a little is it stand alone computer or a laptop?, does any text appear on the monitor ?

The missing information means that we do not have a clue what you are talking about, we need a LOT more exact detail!! Sorry.

I think I know because I've seen it!

I had a monitor that acted dead, but worked fine on another (different) system. Video card was OK, just a bad monitor that would not receive the video signal from the CPU. The original system was DOS, the different was a newer windows system and could overcome this

Showing my age here..

In the bad old DOS days you had to configure the systems
components to work together.

With Windows it's automatic.
It's about the only good thing about it.

you may require driver for the monbitor as your old os as per you was dos and now you are using windows

I think the only suggestion I can make is to either:

1) Throw the monitors away.

or

2) Hand them to an engineer to repair.

John.