Computer won't power up, but PS tests OK

I am new on this board. I have built several computers, but now have a problem I can not solve

I made a mistake connecting a floppy drive power cable to a new USB3 card. I turned the power on, the computer started for a second or two, then nothing.

My computer is dead, no lights, no fan, nothing.

I tested the using a Thermaltake Dr. Power; With the PS connected to the Dr Power PS fan runs and all lights except the -5V, light up on the Dr power.

But, when I reconnect the PS to the motherboard with the 24 pin connector and the eight pin connector, nothing works. The fan in the PS does not go on and there is no light on the MB

I swapped the PS from my wife's computer to test in my computer. Same story, except now when I put her PS back, her computer will not work with her original PS or a brand new one. Same symptoms as above. The PS's test OK, but when connected to the MB, nothing.

Now I am really in trouble. Neither of our computers run.

Any suggestions. Is my original PS bad? Did I fry both MB's?

Thanks for any help
 
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