Make sure the monitor is on.
The light on the front of the monitor should be green to show its on and receiving a signal from the computer.
If the light is green, try turning the monitor off, and then back on.
If the light is not green (i.e. its on but it is orange, red, or blinking), check that the monitor cable is attached to the back of the monitor and connected to the correct connection on the back of your computer. Some computers will have 2 connections that the monitor can be plugged into, if this is so, then try connecting the monitor cable to the second connector.
This may also be due to some problems with the components of the computer. Mainly, the CPU, memory or video card not being seated properly in their respective connectors on the motherboard.
Is the computer set to 110V/115V? On the back of the computer next to the power cord may be a switch that can be set from 110V/115V to 220V/230V. Make sure it is set to 110V or 115V.