PC problem powering up

hard to tell what's wrong unless you have known good parts [like processor, memory, vid card, mainboard]to start swapping out.

If your inspection reveals that you have burnt spots on the motherboard in areas of the capacitors or other electronics,then you can assume that the motherboard is the problem and shorted out for one reason or the other.If some parts ot the MB is fried then start looking for a new board

It's rare for a power supply to fail that drastically; it takes a dead short to burn up components in one like you describe. Did the new one smoke when you installed it, too? The symptoms you describe definitely point to a toasted motherboard, and you might want to recheck the mounting. This often happens if the backside of the motherboard is not adequately separated from the case - a little vibration can make the circuit paths exposed on the backside touch the metal case, and the whole thing fries. When you install the replacement board be sure to use standoffs that lift it away from the surface of the case. Sometimes it's also helpful to put a sheet of non-conductive material (clear plastic sheets available at stationary stores are handy for this) between the board and the case. It's possible, though unlikely, that your CPU and RAM are still functional, so start with just a bare motherboard first to save a few bucks.

Usually burned parts on the board in the power supply indicate a shorted capacitor, or a power surge, during the night.
Since replacing the Power supply did not fix the problem, this speaks for a power surge.
Depending on the severity, only the MB may be bad, and all of the components, (RAM, CPU, VideoCard, Etc. ) may be good, or in bad cases (Direct lightning strike on the power line) everything will be bad.
Your case does not sound very bad, (The Light on MB), so this is probably only a MB problem.
Regards, Chris

Anything could have shorted the power supply from a huge surge in the power line to a failed diode or capacitor. The surge would come from a power outage which when is turned back on by the electric company the initial surge in power goes way high. If you do not have a surge protector then you can damage components on the Mother-board.
Start by removing every circuit board from the computer, except minimum memory and use the onboard video, then when you fire it up press whatever key to go to BIOS setup. If it comes on the your mother board is probably fine. Start adding back circuit boards one at a time and restart each time. You must turn off to add boards back each time!! When the machine fails in this process then that is you defective board that killed your last power supply.

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