Computer has power but will not turn on

[Ultima]

My computer has power but will not turn on. All the leds, fans, hard-drives are working, I have tried to disconnected all cables and reconnected the power cord but didn't solve the problem.

[Rawko]

The green light on the power supply will only detect the existance of electrical current. Ensure your motherboard is on standoffs and insulated from grounding itself. Connect the power cable to a different outlet and/or surge protector to be sure that the problem is not with the computer itself.

[RoninBlade]

Computer has power but will not turn on that means connections not connected properly or Incorrect power supply or Bad power supply, power button, and/or power board or inverter. If the Charge indicator is ON then the adaptor must be good. If it does not charge then either the battery is faulty or there is a problem with the motherboard.

[deoWo]

If your computer displays an error message within a few seconds of starting, you probably have a hardware configuration problem. If you see the Windows logo, you need to troubleshoot startup problems. If you see a "Non-system disk or disk error" message, you need to troubleshoot disk errors. If you don't even see the startup screen, you likely have a hardware problem.
Check this for more information : How to fix a computer that won't start