PC won't boot is PSU bad or is my mobo bad?

I was called to fix a dead computer. The computer wouldn't boot so I assumed it was the PSU. Took out the PSU and put one in that I know worked. Turned the PC on and no problem PC boots fine, everything works.

I had a third PSU sitting there and wondered if it was still good so I pulled the PSU out that I just replaced hooked up the third PSU It didn't work. Ok so bad PSU now I know. So I put the PSU back in that was just working plugged everything back in now computer won't boot? Green light is on at the back of PSU I also have a green light on mobo. I should have just left it alone when I had it running but.......Could the power supply just die like that or did my mobo just die can't figure it out. Can anyone help me please?

I would say the motherboard has gone bad... That "bad" power supply may have taken it out

.

Yeah thanks that's what I thought but was hoping for a different answer.

Are you sure that the last PSU that you've putted back was the one which is running?