Disconnect the power cord form the power supply, be sure the power supply is set for 110 or 220 volts depending on your location and power and double check it (most have a 110/220 switch) Feel/look at the back of the power supply to determine if the fan is working. http://pcsupport.about.com/od/toolsofthetrade/f/powersupplytest.htm
If the fan is working, try another power supply anyway. Plug the computer directly into a known-good (a lamp works) power outlet Check to be sure you do not have a motherboard stand-off in the wrong position and shorting-out the bottom of the motherboard. Inspect/shake and listen for loose metallic objects (loose screws) on top of and under the motherboard and in expansion board slots. Look carefully at the ISA and PCI slots, see if any of the contacts got bent/shorted-out. Sometimes an expansion board will dislodge one and it will be pushed into the bottom of the slot. Inspect the motherboard for broken or burnt components. Carefully inspect the motherboard for black soot from bad bearings and clean and replace the culprit. Look for bent/shorted pins on the motherboard headers and straighten. Be sure the speaker is plugged into the motherboard. If you hear beeps. Decode the beep code. If not, double-check all jumpers. Push down on all chips that have sockets in attempt to reseat them. What CPU are you using? Be sure the core voltage is correct. Be sure the CMOS battery jumper is in the correct position. Some distributors purposely ship motherboards with the jumper in the wrong position. Find the jumper that clears the CMOS, put it into the clear position for several minutes, put it back in the normal position, plug-in the power cord, and push the power-on button. If you apply power to the motherboard with the jumper in the clear position you may damage the motherboard. Pull all boards except video. Disconnect all cables going to all drives, pull all cables except power, power on, and speaker, connect the power supply to the motherboard (the black wires go in the middle on AT power supply connectors--"Black OK, red your dead"), reseat the memory, plug-in and screw-down the display adapter and nothing else (push down on the top, front of the adapter and make sure it is properly seated by looking at it all along the PCI or AGP connector), connect the power-on switch and the speaker. Check the monitor plug for bent or pushed-in pins, connect the monitor and nothing else. Check the monitor power cable. Reseat/replace the memory. Check for Motherboard Electrolytic Capacitor Failures
Check the CPU for bent pins. Try another processor. Note: If you apply power to a motherboard with an Athlon or Duron processor without the CPU fan connected, even for a few seconds, you will fry it (see http://duxcw.com/digest/guides/cpu/socketa/heattip.html
). Check the CMOS battery with a multi-meter. Should be around 3 volts (2.8 is ok). Try a different video board. See if the CPU and memory will work with another motherboard. Pull the motherboard, set it on the box it came in, install video, memory, CPU, power, and power on. See if it boots. I have seen several instances where this works when the board will not work in the case. And, when reinstalled in the case, it continues to work. I have also seen where it didn't work out of the case immediately, but did work the next day and continued to work. One of those mysteries. Temperamental computers with minds of there own. Replace the motherboard.