It sounds as though the red colour gun has gone in your monitor as red appears as black. Try the monitor on another computer if possible to see if it still has the same fault also if the VGA cable is separate you could also try replacing it, but it seems as if you need to replace your monitor.
I agree with SteevScotland, probably a connection problem of some kind - the cable is quite heavy and can drag and pull with surprising force, esp if you move things around. You could also try resetting your monitor to its default settings (don't know how with you monitor but there should be that option somewhere in the settings menu). A long shot, but worth trying if all else fails, is to run the degaussing utility (gets rid of unwanted magnetic build-up on the tube) if your monitor provides one. Good luck. LC.
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