Fans are not spinning or anything? Here's some troubleshooting to help you with a computer that refuses to power up.
In most cases, if all you have is a motherboard and a power supply, the PC should still power up. I've found there are some instances where at least a CPU needs to be present, but if worst comes to worst, you can usually get the machine stripped down to this point and get some sort of POST or figure out if the motherboard or power supply are completely dead.
We'll tackle stripping the PC down as a last resort. The first thing we want to check is the clear CMOS jumper. When an ATX motherboard has it's CMOS jumper in the clear position, it will likely not power up. I'll also have you know that many motherboard manuals I have read have been WRONG when describing what the normal position is on a CMOS jumper, so when in doubt TRY THE MOTHERBOARD WITH THE JUMPER IN BOTH POSITIONS.
The manual not only may be unclear about the clear CMOS jumper, but also where your power switch may go on the board. If you're using an ATX motherboard and case, it's not like an AT case where a mechanical switch turns the PC on and off. The power switch on the front of the case actually shorts two pins on the motherboard that it turn "tells" the power supply to turn on. Double check where you have this power switch hooked up and if you're certain that it is hooked up to the correct two pins, and there is no polarity on this switch, you could essentially remove the switch and by pass it's function by touching the two pins together with something metal (the tip of your screw driver, pliers, knife, etc.).
If this does not work, the next thing to check is your IDE cables. A dead drive or a backwards IDE cable can prevent a board from powering up. There's juice going through those IDE cables as well as data signals, so for troubleshooting purposes, disconnect all of your drive cables, both IDE and floppy.
If doing this worked, then it might be safe to assume that one of your drives are bad, or perhaps a bad cable. Check the way your drives are plugged in. Make sure that the red stripe on each IDE cable is towards pin 1 on each IDE drive and each connector on the board. Plug the drives back in one at a time.
If your PC is still not powering up, then we need to pull all of the cards out of the machine as to make sure that there are no cards grounding out the PC. At thins point, we should only have a power supply, a stick of RAM and a CPU in a motherboard mounted in a case.
Still no power? Wow man! You've got issues!
The next thing we need to check is the board itself. Pull the motherboard from the case. Maybe it's grounding out somewhere. Perhaps there's an extra stand off screwed into the case. What's a stand off? You did use stand offs, didn't you? Try the motherboard, still stripped of it's cards, outside of the case sitting on just a box or table top or anything that WOULD NOT conduct electricity. Double check that CMOS jumper again too!
If the motherboard is STILL not powering up, then we know, at this juncture, that the problem is either the motherboard or the power supply.
To test the power supply, you can do this: Unplug the power connector from the motherboard. Connect the green and black wires on the power connector with a paper clip or a piece of wire. If the power supply fan kicks in, the power supply is probably good and your problem is likely the motherboard. If the fan on the power supply remains still, you may just have a dead power supply.
NOTE: Many boards today have a "standby LED." Just because this is lit doesn't mean that the PSU and/or motherboard is working. It only means the motherboard is receiving +5VSB from the power supply.