If the monitor is showing “No VGA (or some other) Cable”, check the Input on monitor button (varies from monitor to monitor). Change the setting to Auto.
If the monitor shows “Entering Power Save Mode”, unplug the monitor from the computer for a couple of minutes and plug it back into the computer. Make certain that all connections are tight at both ends.
(Many monitor issues are the result of incomplete/loose connections.)
If the monitor comes up then fades after being turned on, the monitor is going bad.