The computer screen will have a bluish, pinkish, or greenish tent. The screen may have lines going through it or it may just be a black screen. As with externals, there are 3 possible causes: The monitor itself is bad (rare), the cables going from the monitor to the computer are bad (common), or the video card drivers needs to be reinstalled (uncommon).
To Resolve: Edit
1. Check the wiring thoroughly. Reseat the cable going from the monitor to the computer on both ends. If it's on a splitter and one isn't working, see if it works when plugged into the opposite slot, could be a defective splitter. Try bypassing the splitter altogether. Hit the "Menu" button on the monitor, if it comes up you know the monitor itself is probably not the issue.
2. Switch the monitor with one that works just for troubleshooting. This will rule out a defective video card.
3. Call the monitor manufacturer for further troubleshooting or replacement.