My monitor is showing black screen with no beeps

What things, cables, wires or parts should I check in a computer that shows only a black screen with no beep sounds with the power on, monitor lights on, and CPU running okay? The CPU led lights are on. The monitor lights up when powered on but turns to blue screen. The system was okay two days ago. It was booting up and was working well. Sorry for repost as I was not a member earlier.

