Could be one of several different things.
The two most likely are a bad power switch or the system needs a PMU reset.
To reset the PMU:
Open the case.
Examine the motherboard for a flat button about 1/2 the size of a pencil eraser. The button is in different places on different motherboards - I don't know where it is on your particular system.
The button is probably grey, probably near the battery, and should be mounted on a small silver square.
Push and hold the button. (Count to 10)
Let go of the button.
Try to turn on the system.
My quicksilver dual 800 refuse4s to turn on.... power light comes on it makes all the usual noises etc hard drives fire up as well. but nothing on the monitor. it has 10.4.3 on and 1.5 gb ram.
i have tried the pmu switch to no avail...
the led on the logic board lights up.
the keyboards capslock light does not.
Have you verified the monitor and video card are good?
Is all the RAM OEM, or have you added it after purchase?
If the latter, start by removing the ram you added.
If you have 3rd party expansion cards, remove them.
Are the mouse/keyboard connected directly, or through a USB hub, including hubs in a monitor?
If you have a Pro/optical mouse, does it's light come on?
The system should boot w/o a keyboard connected.
It's possible the keyboard or USB port is dead. Try the other port.
Yep I tried all that stuff, but thanks for the suggestions.
I think either the Logic Board or the Memory module is dead as there is no light on the capslock or my mighty mouse.
The LED on the Logicboard lights up and the hard drives spin up, likewise the monitor lights up but the screen is blank.
So I think its off to Apple for me...
It refuses to boot again....
It can supply power to the mighty mouse, but not the keyboard (although if you boot up and hold down the mouse button to eject a CD it will power the keyboard)
Everything powers up Hard drives etc, but no visuals on the monitor ( a Sony CRT) although it does turn the monitors power light green.
Any more suggestions guys?