I am assuming this is a older computer( ribbon data cable, NO sata, etc. Disconnect all cables. Make sure power supply cables are plugged in right to motherboard. Did you take out the Processor? If so did you put it back in with a heatsink and use the proper thermo past between? Are the power cables for the processor installed correct? Now, plug in hard drive to proper place( end of cable) on ribbon cable then ribbon cable to motherboard. By the way, what is the HD jumper set to( cable select?). Plug in a single CDrom drive to the data cable. Is it on-board video and sound? Plug in a monitor, keyboard, mouse, and one stick of memory. With this you should be able to boot into the BIOS of the motherboard. Have you check on-line for how to get to the BIOS of your MB? F8, F1, F2, ecs key or the Delete key?
Here's a good diagnosis website.http://www.fonerbooks.com/pcrepair.htm
Now, let's learn what your supposed to do, OK?