Possibly a dead CPU at the worst or faulty/unseated RAM or BIOS has been altered or CMOS battery has died fast.
You say Motherboard isn't powering up?... when you you turn the computer on do these things happen
does the monitor show a " No Signal" message on screen and the amber power light on it not turn green shortly after it makes a click sound , as well as the 3 LED's on the keyboard not glow green ?( the answer to these 3 questions will reveal a lot)
If the first of these three things happens you could have a power supply problem with your computer, check/swap cables that supply power to the com and monitor. ( it COULD be the CPU)
If none of the other 2 things happen that can be a burnt out CPU., OR RAM problem ( do you hear any 'beep sounds' when you turn com on? If you do then you have a RAM problem)
does the power indicator LED on front of computer glow green
does the Power supply fan at back of computer spin
Assuming your power supply fan spins up and you have a green light on your computer and an amber light on front of monitor but no signal coming to it, then do this.....
Take the side off your computer so that you can see the insides of it, does the cooling fan on top of your CPU heatsink spin and does the fan on your Graphics card spin ( if your vid card has one)?
Do the Hard drives spin up? put a finger on one of them, you'll feel vibration
If none of that happens, you have a power supply problem on your motherboard ( ATX cable connected firmly from PSU to it? ...PSU= power supply unit) if so the prob is from that point onwards
If all is spinning and you don't hear 'beeps' then motherboard has power and the RAM is OK. Beeps mean that the RAM is not seated properly or has failed. You could remove the RAM from its slots and reseat it to be sure even if there are no beeps.
Assuming there are no beeps, fans are spinning, the monitor won't display any boot screen or whatever your set up is, no lights on the keyboard and monitor LED is amber and possibly something I have missed out, I would say that I would try another Graphics card and or monitor, no spare vid card try the onboard monitor outlet ( if your com is set up to use it) and try a new or known good CMOS battery.
I'd also try resetting the BIOS by moving the jumper on the BIOS reset pins on your motherboard from pins 1 & 2 to pins 2 & 3 for a while, half hour say, then replace the jumper onto pins 1 & 2. Have a look in the computer manual to find the reset pins. they will be very close to the cmos battery.
Oh, and do that with a good CMOS battery installed and the mains power turned off. Unplug your computer is best to do this.
After all that, still no joy,...IT begins to look like a CPU problem,...check that the fan/heatsink are fixed firmly in place on top of the CPU. If they come off the CPU lifespan about instant death time.
Could also be capacitors burnt out on mobo, of a chip or...or...or
I'd look at it in this order..power supply, monitor, graphics card, RAM, CPU, motherboard.
Hope that helps, sorry I'm a bit long winded.
I've been having the same problem as well. But I figured out that my current memory was to weak so I bought a 667mhz 2gb stick. I installed that and I still have the same problem. Everything is working except my monitor (which has absolutely nothing on it and also the led is orange), the keyboard and the mouse don't light up at all. The fans inside are all spinning but nothing on the monitor. Any help would be appreciated, thanks.
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Do you have a video card? try to check the back panel of your system unit maybe you u plug the monitor cable in the wrong port. if it is correct try to remove your video card and your memory reset your cmos.. turn it on without video card and memory then after several seconds turn it off.. then put back the memory and the video card.. be sure to give enough pressure in installing your video card and memory for preventing lost contacts..