1. Make sure everything is plugged in correctlyThis is a very common mistake. Usually it is the motherboard or the boot drive. Some motherboards have two connectors: The 24-pin one, and a small 4-pin or 8-pin square one. If your motherboard does not have both of these, it will only have the 24-pin. Make sure that is secured into place. If that's ok, move on to all/the hard drive cable(s): 4-pin molex/sata power and the L shaped data (if sata) 40-pin if Eide. The data ribbon should trace to the motherboard. Also,if Eide, the red side of the ribbon should be on both pins 1 or both pins 40; it cannot be turned around.
A general review of all the wires should be performed. Where do they start and end? Do the connections make sense? Despite what most people think, the inside of a computer is really common sense; everything fits only in one slot/hole.
2. Clear the CMOS
This is often the solution, and it's usually bloody frustrating because it's so bloody simple once you know about it. All you have to do is find out from the motherboard manual where the CMOS jumper is. Make sure the system has no source of power (meaning the power supply is unplugged and the battery is removed). Then, move the CMOS jumper over the pins that clear it. After a few moments, put the jumper back, plug the power and battery back in, and try it.
If it works, job done, congrats, fireworks will light the heavens and so on. If not, carry on down the list.
3. Strip the system down
The first thing you should do is remove the system from the case**, place it on a non-conductive surface, and disconnect all components from the motherboard with these exceptions
CPU (and heatsink/fan)
A single stick of memory
Graphics card (if you have one)