Are you getting any juice? As silly as this seems, the number one cause of trouble is not plugging the system in or plugging into an outlet that has no power. One related item to check for is the back of the power supply. Sometimes there is another power switch on it that will override the switch on the front of the case. If your power supply has one, make sure it is in the "1" (On) position, not "0" (Off).

Are you hearing any beeps? If you here a series of beeps from the system speaker, these can indicate a hardware issue. The most common error code is the memory error (usually one long beep followed by three short). If you hear this, re-seat all memory chips and the video card since both can produce this code.

Are the drives powering up? Check the molex connectors on all drives if they don't seem to be doing anything. While you're in there, verify the IDE connections and check all jumper positions on the drives.

Did you hear a pop or smell smoke? This can be a bad one. You've hooked something up wrong and created a short. You may have damaged your hardware. Verify the power connections to the motherboard and all devices. If you don't see a problem, seek assistance before attempting to power the system up again. You are probably not seeing a serious problem, such as incorrect USB connections. If you find incorrect wiring, verify your changes and attempt to power up. If you're lucky, no serious harm was done.