Hi,
The first thing I would do is just reseat all the cables, connectors, and expansion cards and try booting. Make sure you have all the power cables plugged into the motherboard as per the instructions that come with your system board. Try resetting the BIOS if you havent done so already as well. To do this, unplug the computer and then remove the coin battery on the system board. Then hold the power button down for about 10 seconds or so to discharge the board. Then put the battery back in and try booting again. If it doesnt help, then I would try doing a minimal boot to see if you can get anything to come up. Try booting with just the cpu(and its fan), 1 stick of ram, and the video card to see if it will POST. Disconnect all other drives and peripherals. If it comes up saying it can’t find the os or keyboard, plug in devices one at a time and try booting to see where it fails. If nothing comes up on the screen during the minimal boot, it means the problem is with either the memory, video card, powersupply, motherboard, or cpu. Try swapping out the ram with another single stick, and try it in the different ram slots if possible. If that doesnt work, try swapping out the video card with a known good one, or try the onboard video if the motherboard has it. If those dont solve the issue, I would look at swapping out the power supply next, then the motherboard, and lastly the cpu.