1. I would first check to see if someone hadn't unplugged the computer or switched off the switch on the power suppy.
2. Then I would check the AC power with a meter or a lamp etc... to see if the power was indeed good.
3. Take the cover off the computer, disconnect the power supply from the main board and all drives, short the Green wire to a Black Wire on the ATX connector to see if the power supply fan works.
4. If the fan on the power supply will not spin, then I would assume dead power supply. Try another one.
5 If the fan spins, then hook up all the drives, short the wires and see if the fan spins again.
6. If the fan doesn't spin, possible bad drive... unhook all drives again and hook them up one at a time till you find the one that is pulling the power down.
7. If the fan and drives all spin up then hook the power supply back up to the motherboard and take out all the cards except the video card (If there is onboard video, then remove all cards...)
8. Power up the computer with the power button and see what happens.
9. If the computer will not power up, possible bad power button, short the power pins on the mother board momentarily to see if the switch is bad.
10. Take all the RAM out test again...
11. If you still have no power at this point, you will need a spare CPU, Motherboard etc to continue testing.