Try to boot up again. If still no response, the first places to check would be power supply and motherboard. Even though the LED lights, there could still be one of the power leads not powering on. If you have access to another power supply or tester, I would start there. Then if that is OK, and no other items are plugged in, try the motherboard by itself. Of course it cannot proceed with nothing in it, but then if your are successful, start adding hardware until it fails or try a new or known working video card and install that so you can see what the BIOS says. Then try adding hardware one at a time until you get a failure. It would be much easier to try your hardware one at a time in a working system, if you can get someone to agree to that, however. Of course you stand the chance of taking out the other working system with that approach.
This morning my PC booted without problem. After a few minutes the screen momentarily went blank then the desktop was displayed as before. However, there was no response to mouse, keyboard or power down button.
On disconnecting the mains and switching on there is no display at all. All fans run - case, cpu, graphics card and chipset. The system power LED is lit. The DVD drive lights flicker as usual and the drive spins. No hint of hard drive activity no network port leds.
System Board - Asus A8N-E
Processor - AMD Athlon 64 X2 3800
Memory - 2048 MB Corsair
PSU Antec NEOHE 430 GB
Has anyone any suggestions as to the diagnostic approach please?