I have a hitachi 42 inch plasma tv about 4 yrs

Well, composite is still an analog signal format. What input does the monitor go to and is there anything plugged into that input? This is a fairly industrial model and has some features that you would not expect a normal tv to have. Check to see if there is some sort of default input selection in the menu or something along those lines. I have used these units before but I'm not familiar with the overall workings. I know there is signal sensing software on the monitor, for example it will turn itself off if no signal is detected for a while, 10 min is default I think. The issue is more than likely a setting that was changed. One other thing you can try, there are cards in the slots in the back. Put the signal you want to come up in the card slot of the input that it's going to automatically. This is a workaround of the problem, but it would do what you want it to.