Greetings, I'm Adam, I'll be helping you solve your issue!
Is this on a laptop?
Have you tried any other inputs on the LCD monitor (if it has more then one)?
Also, can you try another computer with the screen to make sure it is not a computer issue?
What is the model number of this MAC desktop?
Most have the ability to connect to another video source VIA a RGB cable but I will look that up when I know what MAC computer this is.
I will opt out so any other expert can try to help you. You will receive an email when another expert takes your question. The new expert will see all our previous thread, so no need to explain the issue again.