@warrenisit - First, enter the BIOS: a) is the hard drive correctly identified?, b) load "setup defaults" or similar for your BIOS.
1) remove (disconnect) everything from the computer except keyboard, mouse and video; remove any other expansion cards (leave the memory!) and all peripherals. 2) remove and re-seat video card, if using. 3) remove and re-seat memory. 4) remove and re-connect hard drive connections - at hard drive and at motherboard; also, verify jumper settings, if applicable. 5) verify that all motherboard connections are correct and tight and that all fans are connected. 6) remove and re-seat cpu. You can attempt to boot after each step.
Still no boot: 1) if using a separate video card and the motherboard has onboard video, remove the video card and revert to the onboard video. 2) remove all but one RAM module - verify in motherboard manual what slot to use for one module; repeat using each module by itself, attempting to boot with each module in turn. 3) download and run the diagnostic software from your hard drive manufacturer - you don't need a working OS install for this.
Post back - hopefully with good news!