You've given us NO information about the computer at all. Laptop or desktop? Operating system? Make and model? Tried doing something already to fix the issue? (If you haven't tried reinstalling the graphics driver, give it a try first.)
If the computer is a desktop, try swapping in a known "good" monitor to see if the current monitor is at fault. If the second monitor works fine, then you know the culprit..
If the computer is a laptop, try plugging in a known "good" secondary monitor and see if it displays correctly. If so, then you know the primary display needs to be fixed.
Hope this helps and let us know more.
Help, Please I have lost the reds, greens, yellows in my computer. Don't know what happened.