CHECK SIGNAL CABLE -- the cable between the computer and the monitor might not be tightly connected, or the cable may not be transmitting the signals from the computer.
Or, the monitor is reporting that the computer is not generating any video-output.
This could be a dead motherboard, or a dead video-adapter inside your computer.
Can you connect the monitor to a different computer? Does it work correctly?
Can you connect a different monitor to your computer? Does it work?