If your system does not have a video signal, first double-check all cable connections from your monitor to your system. Make sure they are not loose and that they are all properly connected.
Next, confirm that you are not connecting the monitor to the onboard video output on your system. Make sure your monitor is connected to the video card.
Finally, make sure that you have selected the proper input on your monitor. Most monitors have the feature of selecting the input that the monitor receives a signal from, so changing between input sources may help.