Does the System POST (Power On Self Test) ie do you get hard drive activity and at least one beep from the system speaker.
Try starting the system without the monitor attached and hammer the del key as it starts until you hear the beep, then plug in the monitor. The system should be in the Cmos setup screen which is very basic vga output.
If this work then the problem is somewhere in the power management setup / video card interface.
I have had this problem and am still to find the fix. About to swap video cards for fault finding when I get the 'puter back (inlaws upgrade
)
MAx