See if pressing the caps lock key turns the keyboard LED on and off. If not, it shows the CPU is totally not doing anything.
With nothing connected to the motherboard other than a 5V supply and a CPU board, it should boot into the purple insert disk screen.
When you put the A3640 CPU card back in, make sure you set the two clock source jumpers to the correct position (I think it's external mode for A3640?).
Could be almost anything at fault, but you can narrow it down by thinking what you did in the time between when it was working and when it wasn't.
