Ok, so for some strange reason I have the same problem with two different sets of hardware. The first one I took apart and put back together. When I press the power button nothing happens, absolutely nothing. The green light on the motherboard is on. Same with second, no movement when I press the power button, but again the green light on the motherboard is on.
Both computers were previously working. The second was a brand new setup that I built myself. It would shutdown just after getting into windows. When I looking at the CPU temperature after the shutdown it was about 60C. I thought this was way to high for just loading windows so I decided to look at the heatsink and cpu to see if they were making proper contact. I think they were, but when I put them back together it would no longer turn on.
Have I broken the PSU, the CPU or the mobo?

I've been looking through forums for hours. I took the motherboard out the case, plugged in only CPU, power and a graphics card and it still didn't power up.

If anyone has any suggestions I'd really appreciate it.

Thanks.

Two (simple) things I'd check first:

1. Take out and reseat the ram.
2. Double check your connection of the case's power button to the motherboard. That little tiny two pin wire.

Edit: never mind the first one. If your fans aren't even spinning, I doubt it is the ram being mis-seated..... go for #2.

Chances are you may have fried or short circuted your cpu. Either that or your power supply has completely dropped.

If it isn't the power supply or the power button prongs, have you reset the BIOS on them?

One trick you can do is to have just the mobo/PS/CPU/video plugged in and nothing else. Leave the RAM OUT! Then try turning it on properly with the 2 pins on the board. If those 4 parts are working, you should hear a beep code. This beep code should be the BIOS saying you have no RAM. Same effect if you leave the video off as well. It should beep to say you have no video. So if even just the mobo/PS/CPU don't do anything, you can blame one of them.

Assuming you have just mobo/PS/CPU, does the CPU fan spin when turned on? If NO, suspect all 3. If it DOES spin, leave the system running for a few minutes, does the CPU heatsink get warm at all? If it's cold, the CPU isn't being used, suspect mobo/CPU.
If you get no beep codes at all, suspect CPU or mobo.

It's just a matter of logic. For example, if you get a beep code, then CPU and mobo and PS have to be working because the BIOS routines are running. So the problem would be video or RAM. If you get no beep codes (assuming you have a speaker connected!) then obviously the BIOS isn't starting, so it could be the CPU or mobo. But if you get no beep codes, and the fan doesn't spin either, perhaps the PS is in trouble.

Good luck

Thanks for your suggestions guys. None of the fans spin up, and I've tried shorting the power switch pins on the board instead of using the power button. But like I said, before I took the cpu and fan off it all worked fine. So I think it's either the cpu or the motherboard. But like you say it kind of seems like the psu isn't working. So, I'm clueless. Is there a chance if the heatsink is not properly seated, there would be some safety feature that would not leat the cpu start? Would it make a difference if I reseated the heatsink with thermal paste etc.

Thanks again.

if those suggestions dont work get a new bios cell the little watch battery in your motherboard it keeps a bit of current to hold the bios if it wears out (happened to my brother) the mother board is clueless on what to do when you swich it on. so get it replaced cheap solution so dont go and buy new stuff untill you have checked that

If the green light on your MOBO is on, then it's not the PSU. What is your setup?

Well it could still be the PSU even if it has a green light. Or at least it could be one of the rails coming from the PS. 5V, 12V, 3V? Most PC shops have a PS tester and then can check real quick if it's outputting the right voltages.
Go ahead and post the specs of the PCs, what the parts are. And also post the numbers from the PS, namely the output watts.

There is one rather dangerous test you can attempt. And that is to not have a heatsink or fan on the CPU AT ALL! Okay listen. Take the heatsink off and firmly press your finger on the top of the cpu, then set up the power so you can pull the plug or switch it off withing microseconds of turning it on. The point is, if you press the power button and you instantly start to feal heat, turn it off immediately! The heat means that your CPU is doing some work.
If you turn it on and you get no heat at all, even after a few seconds, and the CPU stays cold, it's NOT working, and you could still have a PS/mobo/CPU issue. But if it does heat up, it could be a mobo issue perhaps.

It could be time to just take those parts and get them checked out.

Be carefull with the heat trick, the CPU can overheat and burn out in no time at all!

Frankly, it could still be mobo/cpu/ps, but it's hard to say unless you have parts to swap with. As any of those 3 going bad, could cause similar symptoms.

Good point about the PSU. And that's a neat trick you have there. You could maybe lower the risk of overheating by leaving the heatsink on and touching as close to the bottom as you can.