The posbilites are:-
# Loose cable, or bent pins on cable.
# Loose video card.
# burnt out power for the video card chip on mainboard (smells bad).
# Video card is dead.
The most common cause of this problem apart from a loose monitor cabel is a loose video card.
Remove the monitor cable from the Video card (back of PC),Open your PC case, earth yourself by touching the casing then see if the video card is loose, if so push it in. If not take it out anyway and put it in again, blowing the dust away (or even better use a vacuum cleaner to vacuum all the dirt out of your PC). Make sure it sits correctly.
Put the cable back on making sure it fits tight and that no pins are bent, and turn it on.
If this doesn´t work then try a different video card, even an old one. IF this works your video card is faulty.
More information would be helpfull.
IS it a TFT or CRT (tube) monitor? does the PC run, can you hear the noise of the power fan and hard drive working?
Does it show the Bios code and video card name when it is turned on, before windows starts?