Unplug everything but the front-panel power switch from the motherboard.
Remove the CR2032 Lithium battery from the motherboard.
Wait one full minute (72 seconds to be safe, if the board was built to good engineering practice).
If the motherboard has buttons, press and hold EVERY button for a few seconds.
If the board has a "clear CMOS" jumper, follow instructions and use that.
Press the system main power button for at least a few seconds, and repeat this a couple of times.
Re-attach all cables, etc, to the motherboard, and then reattempt startup. Sometimes (quite often) this will clear out a sticky bit somewhere that prevents a machine from turning on.
Sometimes there is a capacitor still providing enough voltage to keep a bad bit's state. The above procedure will drain the capacitors and clear that bit.
If it still doesn't want to start after all of that, read how to enable your motherboard's backup BIOS if it is a "dual BIOS" motherboard. Luckily, most motherboards are of this type these days, even a LOT of the cheap ones.
