I've just got back from a 2 week holiday to turkey and when i go to power up my computer everything switches on for about half a second, e.g. all fans and lights and water cooling pump.
I've now taken everything out of the power supply apart from the motherboards main power lead, now i get a long beep everytime i press the power on switch which i think is coming from my motherboard (its either that or graphics cards :S)
A failure to start up like this could be a lot of things. MOBO or PSU are prime suspects but don't assume anything yet.
Be sure power is going to 4 pin CPU aux. Reseat the video card and ram.
If neither helps then best way to narrow it down is to rebuild the system out of the case with only barebones parts. If it won't start up with only cpu and 1 stick ram and the video card hooked up then try a different stick of ram in and put it in a different slot. If that fails then try another PSU. Then another video card.
If none of this helps you should probably assume a dead mobo. Keep trying though, often these things end up having surprisingly simple explanations such as a loose connectiion.