Let me provide a little background to the problem. I was having memory related BSODs which I could not determine the cause of. In one of my attempts to fix them, I changed the BIOS settings for the voltage of my RAM from auto, to 2.1 and forgot to change it back. I think in doing so I may have fried my CPU, but I'm not 100 percent sure. The voltage that my RAM was supposed to be running at was 2.1v and my cpu was running at a much lower voltage.
I left my computer on all day while I was out of the house today and when I checked on it tonight the keyboard and monitor weren't working. The computer was still running but the keyboard and monitor were not. The monitor wouldn't come out of sleep mode and the keyboard's backlight would not turn on. However, everything else seems to be working fine. I touched the heatsink and it felt slightly warm. I feel like thats a pretty good indication that the cpu is still good, but I need a sure way to tell which one is bad so I can replace it.
All of the fans on the computer are running, including the HSF. The motherboard light is green and there is no physical damage to the mobo as far as I can tell. I can't tell you if there's any beeps when I start it up because I removed the motherboards speaker long ago. Also, the mouse is still working.
You need the motherboard speaker. With no speaker, if your system will not POST or put characters on the monitor, you cannot tell if you have a CPU, motherboard, or PSU problem.
Fans running just means that the PSU is producing something around 12 volts. There's no indication if the 3.3 and 5 volt outputs work.
The motherboard LED doesn't mean much than the PSU is plugged in and turned on. It's powered by a small independent power supply called the standby power supply. It is completely separate from the main power circuits that actually run the computer.
Full specs would help (including whether you're using two sticks of RAM or four). I did have a machine once that was having the same problem waking up from sleep mode ... it turned out the RAM voltage had been reset to auto, and correcting it solved the sleep problem as well. That's just a stab in the dark, though, unless we know the rest of your setup.
So I'm assuming when you tried getting it out of sleep mode the keyboard wasn't working, and when you're booting now you're using the same keyboard. Have you tried another keyboard? I'm a bit fuzzy but I'm fairly certain a computer won't POST past a keyboard error to video.