Hi there,
sorry to see your having problems and I don't think there will be a quick fix for this. The reality is, when a PC suddenly stops working and the motherboard and processor are tested as fine then you have to go through every other component to find out which one is failing. Please don't assume that just because everything was working fine that it still is. I have faced a similar problem and the final culprit turned out to be a faulty mouse, which I didn't track down until I had spent a lot of money on other parts.
The obvious place to start is to check that all of the necessary power leads are connected to the motherboard and also the GPU if required.
Disconnect everything else from the system except for one stick of RAM and I do mean
EVERYTHING else including mouse and keyboard at this stage. Then try to start the PC.
I wouldn't rely on a home made motherboard speaker and if possible get hold of a proper one to hear the POST Beeps, if any.
If you still have no luck get the PSU Tested as it may not be delivering enough power or use another PSU to eliminate this as the cause. Again, I have encountered issues such as yours with a brand new PSU so don't miss this step by believing your unit is fine.
If the PC does start, shut it down and then add one component at a time, restarting the PC each time to check if it will still BOOT. Hopefully you will eventually find what is causing the problem.