sounds like a stupid question but my sisters system just died, i know the psu is fine i checked it on another system. when you press the power button, the power led goes on as does the cpu fan, nothing else, the hard drives , dvd drives and there is no display output either from the graphics card or the on board one when i removed the graphics card.
Personally, I would check graphics card and memory first. Have you tried clearing the BIOS via the jumpers? As Fruitbat has suggested disconnecting everything, especially graphics or memory modules should bring up error beeps. Just some vague suggestions - beastly things computers! Process of elimination and usually the last thing tried. :-)
On your motherboard close to the flat batter will be a set of three jumpers. The default setting will be a cap on number 1 and 2 pin - no 3 pin will be clear. To clear the Bios, take the cap from 1 & 2 and place it on 2 & 3. (You should have you computer switched off and the power lead disconnected to do this.) Leave the cap on pin nos 2 & 3 for about ten to twenty seconds and then replace the cap on 1 & 2. This will set your Bios to factory setting and you will have to enter the correct information when you reboot the computer. (If you can get into the Bios before you clear it, make a note of the various settings so you can replace them as they were or should be. Hope that helps. :-)