Hi Tree3. There have been a few threads on this theme recently, some have been resolved and some have come to a dead end without resolution. I hope yours ends in success;-)
If there is absolutely no response to pressing the on button, the first thing is to eliminate as many possibilities as possible. Check the obvious, is there power getting to the computer, make sure there is power at the mains socket, and getting through the cable to the power supply, and that any fuses in the power supply are intact.
Disconnect/remove anything from the motherboard that is not absolutely necessary for a first boot, this leaves only the power supply, the power-on switch, processor, heatsink with fan, a stick of memory, graphics card, case speaker. Especially check the power-on switch leads are in the correct position. You can even replace the switch with a pair of wires that you can momentarily touch together to perform the swith-on function. Make sure that the BIOS reset link is not in the reset position, and that there are no shorts between the motherboard and the case. You can rest the motherboard on a sheet of card to make sure of this. The motherboard should now boot up, complete the POST and then report that there is no operating system. If it doesn't, then the problem has to be in the power supply, motherboard, processor, memory or graphics card.
Check that your fans are connected to the correct points. If your CPU fan is not connected to the correct point, the motherboard may decide the fan isn't working, and switch off the PSU immediately to prevent overheating damage.