That info will help toward progress. Now I have more questions.
1. One of the most common errors I see in this scenario is plugging in at least one thing wrong. One especially. Many systems have a power output cable from the PSU that has some SATA and /or some 4-pin Molex output connectors. Often at the end of that cable is a much smaller connector with four holes in it that looks somewhat like a fan connector. So people plug this into a mobo fan header, thinking it has something to do with fan control. In fact, that connector on some power outputs from the PSU is ONLY to be used to provide power to a 3½" floppy disk drive. It should NEVER be plugged into a mobo anywhere! When you do, you are "pushing" power from the PSU back into a header that is trying itself to "push" power out to a fan. This always results in total failure of the mobo to be able to start up. SO, look carefully at the stuff you plugged in when you re-connected everything. FIRST check whether you made this specific error; if you did unplug it NOW. THEN re-check all your other connections to be sure you did them right.
2. Another common error is failing to connect something needed. One is the PSU output cable to the mobo's CPU chip power socket. If you have not plugged that in, the CPU gets no power. Another is for your video card: IF you have a separate video card (not just built-in video on the mobo), it may require a power supply connector from the PSU, too.
3. You say you tested the re-assembled system, MINUS the fans, before connecting those fans. I presume you mean only the case fans - that is, when you tested, the CPU cooling system was in place and working. So, what did the system do? Did it boot completely and start up Windows on your monitor? Or, did you just let it start up part way and then shut it down yourself since you did not have any cooling? Or something else?
4. Your last post says you replaced everything, just about. Especially when you replace the mobo and CPU, normally that would be a problem booting. That is, the machine would start up and complete the mobo POST, then try to boot Windows. There it would fail, often leaving nothing but blank screen, but sometimes showing an error message on the monitor. Did that happen?
5. You changed the GPU - I presume that means you have a separate video card. Plus, you changed the mobo. Very often with those components new, the mobo BIOS is set to use whatever built-in video system it has. Thus its output to your monitor is from the video outputs on the back plate of your mobo, and NOT through the output connectors of your video card. In this case, the system will appear to start up in terms of things whirling and moving, but your monitor (plugged into your video card) will only say something like "No Signal". Is that what you are seeing?
One important point in assembling a system is the location of stand-offs on the case that support the mobo. You removed and replaced the mobo. Stand-offs are little (often brass) things with a threaded shaft on one end that screws into holes in the back plate of the case. Then each also has a threaded hole in it, and you place screws through holes in the mobo and tighten them into the standoffs. Now, many mobos have their mounting holes in the same places, so just changing to a new mobo may not require changing the stand-offs. BUT you MIGHT need to do that. What is important is this: your must NEVER have a stand-off under the mobo in a place where there is no matching mounting hole in the mobo. If you do have that mis-match, the unused stand-off becomes a source of shorting out a mobo trace to Ground, and that causes total failure. Of secondary importance is that there really should be a stand-off under every mobo mounting hole so that the mobo is properly supported everywhere. So, re-examine how your mobo is mounted in the case. Look especially for a place where there is no screw in a mounting hole. That would be a good hint that one stand-off is missing, and MIGHT be in the wrong place! To be absolutely sure, remove the mobo from the case and examine carefully the location of its mounting holes (usually there are nine) and compare that to the location of the stand-offs screwed into the case back plate. If any are out of place, just unscrew them and re-locate to a place that matches a mobo hole. By the way, you will see that each mobo mounting hole is surrounded by little metal fingers. THOSE locations ONLY on the mobo are designed to be grounding points, so it is proper for metal screws through those holes to connect the mobo to the case metal.
Let me know about these points, and we can proceed if it still is not working.
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