Please post your system specification, makes, models and a more detailed explaination of what has been happening if you can.
The PSU wont work unless it can detect the motherboard
Since both the motherboard and PSU have been changed out, what about the CPU?
Even with just the PSU, motherboard, PC speaker and CPU you should be getting some sort of indication that it is alive. Verify that you have the internal speaker connected to the right pins on the motherboard.