|The power saving message was probably generated by the monitor and not the PC.|
Do you get anything on the screen other than the power saving message like a posting screen or gateway logo? Does F8 bring up the boot menu?
If 'no' then continue:
When you say it's 'on' you mean the fans and drives are spinning and maybe the LEDs are flashing. That only means the power supply is giving 12 volts. A power supply must provide several voltages at sufficient amperage for the PC to work so it can be bad and yet the fans and drives will spin.
But first ascertain if it could possibly be one of the peripherals. Remove all the cards except video and disconnect all the drives. Then see if you get a posting screen. If you do then one of the disconnected items must be at fault.
If possible, swap the video card with another--even a cheapie--just to see if you get a posting screen. Or add a card if you have on-board video.
If you still get nothing, it's probably the power supply and/or the motherboard. It's often both as a failed power supply will sometimes ruin the motherboard.
Now that's what I call a sticky situation