|There are any number of possible problems that can exhibit your symptoms. Basic troubleshooting has been described many times in these pages.|
You have not given any details of your system's hardware specifications or even if it is a brand-name system or was custom built. Do you get any POST beep when you try to startup?
To get a system to POST you need only the following components connected:
Powersupply, Motherboard, CPU with heatsink/fan. One stick of RAM, Graphics card ( Use onboard if available) and Keyboard.
Disconnect all drives Both power & data cables. Disconnect mouse and any USB devices. Remove any additional RAM and any PCI cards.
Now try to power up. Any Beeps? If NO then remove remaining stick of Ram and retry. If you still get no BIOS Beep Error then you have narrowed the problem to one or more of Powersupply, CPU or motherboard. If you do get beeps that at least confirms the CPU and Motherboard are working.
The fact that fans spin & LEDs light up does not guarantee your powersupply is fully functioning. It supplies voltage on several different rails and one can fail while others continue to work. You said your fan spins. Which fan? The one inside the powersupply? The one on your CPU Heatsink or a casefan? The two critical ones are the CPU and PSU fans.
Now if you do get BIOS beeps with all RAM removed, the next thing to try is to return a stick of RAM and remove the graphics card if it is a PCI, AGP, or PCIe card. Try again to power up. What result do you get?
There is always the possibility that in Barebones configuration the system will boot up and allow you to get into BIOS. If this happens then you know that your problem lies with one or more of the peripheral devices you have disconnected and it is a simple matter of elimination to determine which has failed by reconnecting them one at a time and trying to boot after each change.
Goin' Fishin' (Some day)